Perhaps I'm misunderstanding something here, but you might also want to check out your hard drive. Hard drive errors can cause files to become corrupted quite easily, and probability of file corruption increases as more data is written to the drive (eg. after formats).
Actually, sorry, now that I think about it, formatting should flag any bad sectors on the hard drive. Check out some
Diagnostic utilities, as some are *very* thorough and they may be able to find the problem with your computer.
Good luck!