What are Windows Installer patch (.MSP) files? Can you remove them?

Windows Installer patch (.MSP) files are update packages for apps installed with Windows Installer, and you should not delete them by hand from C:\Windows\Installer because Windows needs them to repair, update and uninstall those apps.

This guide explains what an MSP file contains, where Windows keeps it, the supported way to remove a patch, how to cap the patch cache, and what to do if cache files are already gone.

Can you delete Windows Installer patch files?

Microsoft's guidance on the Windows Installer cache is direct: the C:\Windows\Installer folder should not be deleted, and files missing from it cannot be copied from another PC because each one is unique.

Problems do not show up straight away. They appear later, when you try to uninstall, repair or update the app that owned the file.

Do not delete .msp or .msi files from C:\Windows\Installer manually. Remove a patch you no longer want with msiexec or the app's own uninstaller, and free disk space with Storage settings instead. Windows Installer reads the cached patch files whenever it repairs, updates or uninstalls a product, and Microsoft support engineers cannot recover missing cache files. In some cases the only fix is to rebuild Windows and reinstall the app.

What are Windows Installer patch (.MSP) files

A Windows Installer patch is a self-contained file that delivers updates to an application that was installed from an .msi package.

It can be much smaller than a full .msi because it can carry only the changed bits of a file, and it keeps the user's customisations through the upgrade.

Part of an .msp file	What it holds
Summary information stream	A GUID that identifies the patch, the GUIDs of earlier patches it replaces, and the product codes it can update
Transform substorage	At least two database transforms per target: one updates the app's installation database, one adds the patching information
Cabinet file stream	Binary patch data, new files added to the app, or whole replacement files
MsiPatchSequence table (Windows Installer 3.0+)	The order the patch applies in relative to other updates
MsiPatchMetadata table (Windows Installer 3.0+)	Descriptive details, including the AllowRemoval flag that makes a patch uninstallable

Unlike an .msi installation package, a patch package has no installation database of its own. It only describes changes to an existing one.

MSI vs MSP vs MST file extensions

Windows Installer uses several related file types. Telling them apart shows why an .msp file is useless without the app it patches.

Extension	Windows Installer file type	Typical use
.msi	Windows Installer database	Installs the full application
.msp	Windows Installer patch	Updates an application that is already installed
.mst	Windows Installer transform	Customises an installation, for example for one organisation
.msm	Windows Installer merge module	Shared component built into other .msi packages
.pcp	Windows Installer patch creation file	Developer input used to build an .msp

Why the C:\Windows\Installer folder is so large

When an app installs through Windows Installer, the files needed to update and uninstall it are copied into the Windows Installer cache at C:\Windows\Installer.

Cached files get short generated names such as 1fdb1aec.msp instead of the patch's original name, so the folder gives no clue which app owns which file. Viewing the folder in File Explorer is harmless; show hidden files and folders if Installer does not appear under C:\Windows.

What builds up	Why it grows	Who controls it
Cached .msi packages	One per product installed with Windows Installer	Needed to update and uninstall that product
Cached .msp patches	One per patch applied, for example each Office or SQL Server update	Needed for later updates and for removing the patch
Older file versions saved by patching	Windows Installer keeps old files so later installs need not fetch them from the original media	The MaxPatchCacheSize policy, 10% of disk space by default

Busy machines with Office, Visual Studio or SQL Server collect many patches over time. That is expected behaviour, not a fault.

What happens if you delete Windows Installer cache files

Action after deletion	What goes wrong
Installing the next update for the app	Setup stops with a missing .msi or .msp error, or error 1635: "This patch package could not be opened"
Repairing the app	The repair fails because the cached package is missing
Uninstalling the app or a patch	The uninstall fails because the cached file it needs is gone
Copying the files from another PC	Does not work, because each cached file is unique to the machine

Microsoft's only listed recoveries are the vendor's own repair steps, a system state backup of the cache, or rebuilding Windows and reinstalling the app.

How to remove Windows Installer patch (.MSP) files

The supported way to get rid of an .msp is to uninstall the patch from the product it updated. Windows Installer then removes the change and its cached file itself.

Start by finding out which product a cached file belongs to. This only reads the registry and changes nothing.

1. Note the name of the cached file you want to identify, for example 1fdb1aec.msp in C:\Windows\Installer.
2. Press Win + R, type regedit and press Enter.
3. Go to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\UserData\S-1-5-18\Patches.
4. Find the subkey whose LocalPackage value is the file you noted. The subkey name is the patch's packed GUID.
5. Go to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\UserData\S-1-5-18\Products and find the same GUID under a product's Patches subkey.
6. Read its DisplayName value, for example a KB-numbered update for SQL Server or Office.
7. Close Registry Editor without editing anything, then remove the patch with the vendor's uninstall route or the msiexec command below.

Uninstall an MSP patch with msiexec

Windows Installer 3.0 and later can uninstall a patch from the command line. Run it from an elevated Command Prompt, and replace the two GUIDs with the product code and the patch code from the vendor's documentation.

msiexec /package {ProductCode} /uninstall {PatchCode} /passive

/package names the installed product by its product code or .msi path. /uninstall names the patch by its patch code GUID or the full path to the .msp file. /passive shows a progress bar with no prompts. The older equivalent is msiexec /I {ProductCode} MSIPATCHREMOVE={PatchCode} /qb.

You should see: The progress bar closes without an error message, and the app's version number returns to its pre-patch value.

Add /norestart if the machine must not reboot on its own, because /passive restarts Windows whenever the installer decides a restart is needed.

Why some Windows Installer patches cannot be uninstalled

Whether a patch can be removed depends on how it was built and installed. For a patch that is not uninstallable, the only route is to uninstall the whole app and reinstall it without that patch.

Reason	What it means for you
Applied with Windows Installer 2.0 or earlier	Patch removal did not exist before Windows Installer 3.0
DisablePatchUninstall policy set to 1	Nobody on the PC can remove patches, administrators included
No AllowRemoval = 1 row in MsiPatchMetadata	The vendor built the patch as permanent
Patch performs a major upgrade	Reinstall the older version of the app instead
Patch applied to an administrative installation	Reinstall from the administrative image
Per-machine app and you are not an administrator	Sign in with an administrator account, unless the vendor marked it as a LUA patch

Limit the Windows Installer patch cache with MaxPatchCacheSize

When a patch is applied, Windows Installer can save older versions of the patched files so later installs do not need the original media. The MaxPatchCacheSize policy caps that store as a percentage of disk space.

The default is 10%. A value of 0 stops new files being saved, but it does not delete files already in the cache.

1. Press Win + R, type regedit and press Enter, then approve the User Account Control prompt.
2. Go to HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Windows\Installer. Create the Installer key under Windows if it does not exist.
3. Right-click the right pane and select New > DWORD (32-bit) Value.
4. Name it MaxPatchCacheSize.
5. Double-click it, select Decimal, and enter the maximum percentage of disk space, for example 5, or 0 to stop caching old files.
6. Select OK and close Registry Editor.

A smaller cache means a later patch install may ask for the app's original installation source.

Windows Installer patch cleanup: safe ways to free disk space

If low disk space is the reason you opened C:\Windows\Installer, the supported tools reclaim space without touching the cache. Start by checking what uses disk space on Windows 11.

Option	Where to find it	What it removes
Cleanup recommendations	Settings > System > Storage > Cleanup recommendations	Temporary files, large or unused files, files synced to the cloud, unused apps
Storage Sense	Settings > System > Storage	Temporary files and Recycle Bin items, on a schedule
Previous version of Windows	Listed under Cleanup recommendations for 10 days after an upgrade	The Windows.old folder; you lose the option to go back
Uninstall unused apps	Settings > Apps > Installed apps	The app, through its own installer
Third-party orphaned-patch tools	Not a Microsoft tool	Moves or deletes cache files Microsoft says to keep; any mistake cannot be undone by Microsoft support

Excessive DMP files in Windows 11 System32 folder

Large .dmp files are a separate space problem from the Installer folder. They are crash dumps written by Windows Error Reporting or by a Stop error, and their location tells you what crashed.

Crash dumps of services that run as System land under the System profile folder inside System32, which is why they appear there.

Dump file location	What writes it	How many are kept
%WINDIR%\System32\Config\SystemProfile (CrashDumps under the profile)	Local crash dumps of services running as System, when WER LocalDumps is enabled	DumpCount, default 10; the oldest is replaced
%WINDIR%\ServiceProfiles	Crash dumps of Local Service and Network Service processes	DumpCount, default 10
%LOCALAPPDATA%\CrashDumps	Crash dumps of desktop apps for the signed-in user	DumpCount, default 10
%SystemRoot%\Memory.dmp	Complete or kernel memory dump after a blue screen	One file, overwritten by the next dump
%SystemRoot%\Minidump	Small memory dumps after each blue screen, named by date	A new file every time

A steady stream of new dumps means a service or driver keeps crashing. Fix that cause first, or the folder fills up again.

Local user-mode dumps are controlled under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\Windows Error Reporting\LocalDumps, where DumpCount sets how many files are kept.

Windows Installer patches vs Windows Update patches

Windows patching and app patching use different systems. Monthly Windows updates are not .msp files, and they do not live in C:\Windows\Installer, so you uninstall a problematic Windows update from Settings rather than with msiexec.

	Windows Installer patch (.msp)	Windows update
Updates	One app installed from an .msi, such as Office or SQL Server	Windows itself
Installed by	msiexec, or the vendor's setup program	Windows Update or Component Based Servicing
Listed by Get-HotFix	No	Yes
Removed with	msiexec /uninstall or the app's uninstaller	Settings > Windows Update > Update history > Uninstall updates
Apply manually with	msiexec /update patch.msp	The update's own package from Microsoft

Install a Windows Installer patch from the command line

Vendors sometimes ship a bare .msp for manual install. Close the app it updates before you start.

1. Select Start, type cmd, then select Run as administrator.
2. Run msiexec /update C:\Path\patch.msp /passive with the real path to the patch file.
3. Add /norestart to the command if the PC must not restart automatically.
4. To log the install, add /L*v C:\Logs\patch.log; the C:\Logs folder must already exist.
5. Open the app and confirm its version number changed.

The older Windows Installer form, msiexec /p patch.msp, does the same job.

View installed patches and update history on Windows 11

Windows updates and Windows Installer patches show up in different places, so check the right list for the patch you care about.

1. For Windows updates, select Start > Settings > Windows Update > Update history.
2. Select Uninstall updates under Related settings to see the updates that can be removed.
3. For a command-line list, open PowerShell and run Get-HotFix. It lists Windows updates with their HotFixID and InstalledOn date.
4. For an app's .msp patches, read the product's Patches subkey under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\UserData\S-1-5-18\Products; each patch has a DisplayName.
5. Check the app's own About or version page; the version should match the latest patch you applied.

Get-HotFix does not return updates delivered by Windows Installer, so an Office or SQL Server .msp patch will not appear in its output.

Fix missing Windows Installer cache file errors

"This patch package could not be opened" (error 1635) during an update

A cached .msp the next update depends on is missing from C:\Windows\Installer.

1. Note the cached file name and the original patch name from the error or the setup log.
2. Look up the patch's DisplayName in the registry as shown above to get its KB number.
3. Download that update from the vendor and extract it, following the vendor's documented extraction switch.
4. Copy the original .msp into %windir%\Installer\ and rename it to the cached file name.
5. Run the update setup again; repeat for any further missing file it reports.

Uninstall or repair asks for the original .msi

The cached installation package for the product is gone.

1. Insert or mount the original installation media for the app.
2. Point the prompt at the matching .msi on that media.
3. If the vendor provides its own repair tool for missing cache files, use it.
4. If neither works, restore C:\Windows\Installer from a system state backup.

A patch will not uninstall

The patch is not uninstallable, or your account lacks the rights for that install context.

1. Run the msiexec command from an elevated Command Prompt.
2. Check that DisablePatchUninstall is not set to 1 under HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Windows\Installer.
3. If the patch is still refused, uninstall the whole app and reinstall it without that patch.

Windows Installer patch files FAQ

Can Windows Installer patch files be deleted?

Not by hand. Microsoft says the C:\Windows\Installer cache should not be deleted, because Windows Installer needs those .msp and .msi files to repair, update and uninstall apps. Remove a patch with msiexec /uninstall or the app's own uninstaller instead.

What is a Windows Installer patch?

A Windows Installer patch is an .msp file that updates an app installed from an .msi package. It contains database transforms and patch data, so it can be far smaller than a full installer and keeps the user's settings through the update.

What is Windows patching?

Windows patching is installing updates that fix security holes and bugs. Windows itself is patched through Windows Update, while apps installed with Windows Installer are patched with .msp files from their vendor, such as Office or SQL Server updates.

What is patch installation?

Patch installation is applying an update package to software that is already installed, rather than reinstalling the whole program. For Windows Installer apps, msiexec /update patch.msp applies a patch and records it so it can be repaired or removed later.

How do I install Windows patches manually?

For a Windows Installer .msp, open an elevated Command Prompt and run msiexec /update followed by the full path to the patch. Windows updates are installed through Settings > Windows Update, or from the update package Microsoft provides for that KB.

How do I check which Windows patches are installed?

Open Settings > Windows Update > Update history to see Windows updates, or run Get-HotFix in PowerShell. Neither shows .msp patches for apps; those are listed under each product's Patches key in the Windows Installer registry data.

Is it safe to use a patch cleaner on C:\Windows\Installer?

It is a risk Microsoft does not support. If such a tool removes a file Windows Installer still needs, later updates, repairs or uninstalls fail, and Microsoft engineers cannot recover the file. Storage settings free space without touching the cache.

What is the Windows patch install command line?

Use msiexec /update C:\Path\patch.msp /passive to apply a Windows Installer patch with a progress bar and no prompts. Use /quiet for no interface at all, and add /norestart if Windows must not restart on its own.