If you haven’t formatted the partition and installed Windows 8 over it with custom install option instead of Upgrade or after Windows 8 Refresh, then files in previous version of Windows will be stored in Windows.old folder. They do occupy large hard drive space, after using Windows 8 for 1 or 2 weeks if you’re sure you won’t required these files in future then you can safely remove this folder and reclaim disk space using built-in Disk Cleanup tool in Windows 8, here is how.
Removing Windows.Old Folder in Windows 8
1. Use Win+R command to open run box and use Cleanmgr command.
2. Select Windows 8 installed drive and click Ok,
3. Disk Cleanup tool scans the drive and shows shows lists of unnecessary files found, click Cleanup system files
Now Disc Cleanup again asks to select the drive, select Windows 8 drive, click OK,This time the tool scans for previous Windows installation files, displays them if found
4. Select “Previous Windows Installation” and click Ok to let Windows delete them safely.
Update October 25,2013: You can use above same instructions to remove Windows.old folder in Windows 8.1 also. CCleaner also cleans that folder.