When performing a "Custom Install" of Windows 7 / Windows Vista, any files from the previous version of Windows which was already installed will be stored in a Windows.old folder.
The Windows.old file is automatically generated when "Custom Installing" Windows 7/Vista. For example, if I had Windows Vista and clean install Windows 7, all the old Windows Vista files will be stored in a Windows.old file located at:
This file will be huge, because it stores everything from a previous version of Windows. So if you have backed up all your documents etc, you may wish to delete it, but it is not as easy to delete as you think.
NOTE: Copy any files that you want out of the Windows.old folder before you delete it, you can only do this while logged on to an admin account.
If you would like to delete the Windows.old folder, try the following...
1. Click Start, in the search box type without quotes "disk cleanup" and hit enter.
2. Select the drive / partition which Windows Vista / 7 is installed on. ( See image Below)
3. It will quickly scan your computer. Then it will display a window like the one below. You need to click "Clean up system files". If prompted by UAC, then click on Yes. (See image below)
4. It will again ask you to select the OS and scan your computer. (See image below)
5. Make sure "Previous windows installations" is ticked and click Ok. (See image below)
6. Click delete files. (See image below)
7. Windows.old should now be deleted.
Sometimes the contents of Windows.old will be gone, but the folder and some subfolders is still there and undeleteable, because it "Cannot be found".
If this is the case, try the following...
1. Click Start, in the search box type in without quotes "cmd" and hit enter.
2. Type in....
rd /s /q c:\windows.old
and hit enter.
3. The directory should now be gone.
Hope this helps anyone with this problem, I enountered the same problems, this is why I wrote this How to. Feel free to link to this How to from the forums.