Deriks Boot and Nuke at: http://www.dban.org/download Very good utility to wipe a drive. Download, burn the image to a CD, boot off the CD and nuke away.

Edited to add: When drives write information to the drive, it writes it in 1's and 0's. DBAN overwrites the drive with all 1's or 0's essentially making any data unrecoverable. IIRC, it has DOD acceptable capability.

On a drive that might just have the data deleted, generally only the first character of the file is deleted. If you try recover the file, you usually can do so by substituting a character for that first missing letter, and the file is readable.

DBAN overwrites all the characters so it is gone forever.