Most file deletions aren't actually deleting the data on the disk; it just deletes the pointer that lets the file system know that data is there. The recovery tools work by attempting to recreate that pointer information for the still existing data (if it hasn't been overwritten yet). RWipe is a decent tool with a free trial period if you do need to clean a computer and don't want to take a sledgehammer to the hard drive.
http://www.r-wipe.com/