Its simple, just format your HD about 7 or 8 times. I think there are programs that will write random data on your HD to help cover-up your deleted fills.
Get back-up for this idea before you run with it,
If that is a lil bit extreme, and you don't want to give your boss an empty shell, try google. Live-cd OSes should let you fix your problem. Now try to find a good disk cleaning software that will clear the free space on your HD. that should help cover your tracks.