I use Dirik's Boot and Nuke, DoD 5520.22m level wipe. You have the option of 1 pass / 15 pass (takes longer) and 30 pass (takes way longer)
Or you can boot from a linux live cd and "man dd" for more information on the tools generally included in linux live cd's for wipe hard drives.
Or you can boot from a linux live cd and "man dd" for more information on the tools generally included in linux live cd's for wipe hard drives.