I'd do a backup of his data files first, but if I remember right if he does the install into the same directory everything will still be there, but the registry entries won't be, you'll likely have to re-install most of the software anyway, so I'd suggest, backing up his data files (don't forget his favorites folder, and his .pst {e-mail} file) and re-format the drive with a fresh install. It's always more stable that way. I'd only install over the old os if you're in a pinch trying to get to the os in order to backup his data files.(when you can't boot to safe mode because of catastrophic problems)
Cheers, :beer: