Darik's Boot And Nuke | Hard Drive Disk Wipe and Data Clearing
Run quickwipe, or the DoD Short Wipe if you're really paranoid. There are plenty of people here and on other forums who will tell you to run the 35-pass Gutman Wipe "to be safe," but one pass of random data is more than enough to make your data unrecoverable by anyone who is governed by the laws of physics.