Best thing to do is to physically remove and replace the hard disk with a new one, then hit the original with a hammer.
Next best thing to do is to use a freeware program such as
Darik's Boot and Nuke to securely overwrite the entire disk, then re-format and re-install the OS. That will be sufficient to prevent the average user from finding anything, but might not prevent a serious forensic search from recovering files.
Not recommended would be to just "erase" certain files, leaving the OS and applications intact.
SD