When you install Vista on a computer that has a prior version of a Microsoft operating system on it, a lot of the information from the prior installation will be saved in a directory called Windows.old. If you have installed Vista multiple times, you may have more than one Windows.old directory.
Once your Vista install is up and seems stable, you may want to reclaim that space but will find you can’t merely delete the folder, even logged in as an administrator. There is a method provided to remove the directory but I didn’t know it until I stumbled upon it.
To remove the files from these old Windows installations, do the following.
1. Click on your Start button.
2. Click on Computer.
3. Right Click on your main hard disk.
4. Select Properties.
5. Click on the Disk Cleanup button.
6. Click on Files of all users in the popup.
7. Click on Continue.
8. Look through the list of file types and check the box next to Previous Windows Installation(s).
9. Click OK to perform the cleanup.
Hope this is helpful.