You might find this link useful.
http://www.cyberscrub.info/download/
It's for a free trial of "cyberscrub". You can use to to totally wipe (and scramble anything on the hard disk before you return the computer.
Don't forget that deleted files are very easy to recover by someone who knows what they are doing.
I've used it for a couple of years, and found it very useful
If you do download it, I'd advise you to set up a new e-mail address (yahoo or hotmail), or you might find yourself bombarded with junk in the future.