+1 on KillDisk.

You download it as a CD Image (iso) and burn it as a disc image. You stick it in your CD Drive, set the BIOS to boot to the CD-Rom drive first. It will start its own program. Once you identify the drive you want to erase you start it and it performs a low-level format wipes every last bit to a 0. Takes a couple hours depending on drive size.

This is much better than the old FDisk and Format C: from DOS days. All that does is rewrite the boot sector on the HD and rewrite the partition tables. The data is still there.

I've used it more than once and it works wonderfully. Just make sure you have your OS disk to reinstall before you start wiping.

Definitely a CD to keep in your computer repair discs along with a Knoppix-Live Linux DVD.