Darik's Boot and Nuke (DBAN)
I destroy my old data using what storage experts call a "block-erasure utility" such as Darik's Boot and Nuke. DBAN overwrites each block on a computer's hard drive several times--up to standard levels specified for the Royal Canadian Mounted Police and the U.S. Department of Defense.
DBAN can create either a bootable floppy disk or an ISO file that you burn to a CD. Once you boot from the startup disk (you may have to tweak settings in your system's startup program to boot from the floppy or CD), just a few keystrokes will begin blowing away your data.
Note: This program is donationware. It is free to try, but the author accepts and encourages donations towards further development.