OH recommends this freeware application:
http://www.dban.org/You have to download the software onto a CD-RoM (or a memory stick if you don't have a CD writer), and then start the computer that you want to wipe with the disk in the drive. It deletes everything by overwriting it, so that it is all-the-way gone, and not retrievable even through forensic recovery
There are detailed instructions on the website!
He told me to say don't do this unless you are really, really use this unless you are really, really sure you don't need anything off the computer, and you have all the disks/software, keys, passwords and the like to rebuild it