There are dozens of disk-wiping programs freely available - try going to
http://sac-ftp.externet.hu/security12.html and downloading Wipedisk.zip - unzip it and follow the instructions. I've never used it.
If you're that worried why not smash it with a rock? Quick and VERY secure....
48 hours after you have zapped it you will remember a file you wish you'd saved...GUARANTEED!