Instead of a clean install, if you’ve upgraded from Windows 8 to Windows 8.1, then files and folders in Windows 8 would have been moved and kept in Windows.old folder. The files in this folder may come handy if upgraded OS not working properly for you. If that’s not the case and Windows 8.1 is working fine for you then instead of using Windows built-in Disk Clean-up tool, you can use CCleaner to remove that folder and gain gigabytes of disk space and here is how that can be done.

Cleaning Windows.old folder in Windows 8.1 using CCleaner

CCleaner version 4.07 has been released with support for Windows 8.1 and IE11 Release Preview for Windows 7.  This version has added support for cleaning of old Windows Installation folder. 

After installing CCleaner, you can find "Old Windows Installation" entry under "Advanced" section under Windows tab of  program window.

To know how much space that Windows.old folder is occupying, you need to select above said option under Advanced and click on "Analyze" button, CCleaner shows space you can gain by removing that folder. Proceed for cleaning.

CCleaner advanced old Windows Installaltion