I've used autoclave before, which looks to be the predicessor to Darik's Boot and Nuke. I works great by overwriting your drive several times so the information of it is completely unrecoverable. Then, through on a fresh installation of windows and sell the computer, worry free.

Warning: If this program works the same, then the higher settings (3 passes) took 1.5 days to complete on an 80GB hard drive. Just be prepared for that.