In Windows Exporer, right click on that drive, choose Properties and then Tools. You should see the option there.
What happens is that windows gets lazy and doesn't really delete files. It just hides them. So when doing a reinstall, if it finds a file on the drive already, it will just unhide it again. Speeds things up. But does pose a problem if the file is corrupt. De-fragging re-arranges the order in which the files are stored on the hard drive usually overwriting the hidden deleted files.