You can try using CCleaner.
Set the settings to overwrite files 7 times. That should deter
anyone from using free tools to recover files. However, if he's
using a paid recovery software, I can safely say that he will be
able to nearly everything even if you reformat.
Or if you really want the OS to be a clean one, get a new hard disk
and just install XP and nothing else. Remember to plug out your old
hard disk and keep it somewhere. Once he's done, unplugged it, and
plug back your old one.
