How do I delete Windows.old in Windows?
The only reason you might need to keep the "Windows.Old" folder is if it contains files (bookmarks, settings, etc.) from your old Windows installation that you don't have backed up elsewhere or haven't already extracted. If so, I suggest that you do that now.
The reason you have been unsuccessful in trying to delete the "Windows.Old" folder is because you were trying to do it from Windows Explorer. Apparently this is forbidden. The steps that you need to follow to delete this folder are listed here (note that you may have to run Disk Cleanup as an Administrator—right-click on the shortcut and then on "Run as Administrator"):
1. Open Disk Cleanup by clicking the Start button. In the search box, type Disk Cleanup, and then, in the list of results, click Disk Cleanup.
If you're prompted to choose a drive, click the drive you just installed Windows on, and then click OK.2. In the Disk Cleanup dialog box, on the Disk Cleanup tab, click Clean up system files.
If you're again prompted to choose a drive, click the drive you just installed Windows on, and then click OK.3. Select the Previous Windows installation(s) check box, and any other check boxes for the files you want to delete, and then click OK.
In the message that appears, click Delete Files.
If you're a command line junkie, you can also take ownership of the folder using takeown, but if you're really a command line junkie, you already knew this.
You need to take ownership before you can delete it. Try the following one liner in an elevated command prompt:
takeown /F C:\Windows.old\* /R /A && cacls C:\Windows.old\*.* /T /grant administrators:F && rmdir /S /Q C:\Windows.old\
This will take ownership of the folder, if that is successful, modify the DACL to grant administrators full control of the Windows.old directory and all of its subdirs, if that is successful, then remove the directory.
In future you may just wish to do a clean installation, Windows.old is designed to be a backup of the previous installation, this will obviously not occur if you do a clean installation, either through the setup or by reformatting the system partition prior to (re)installing Windows.
Windows.old contains all your old Program Files, Windows, and personal spaces (C:\Users\your name\etc).
You can delete it (you may need to take ownership of it first, and reassign your rights to it). But like i said it contains your old desktop and personal folders, you may still have stuff in there that you want.