My favorite method of cleaning your hard drive is at this link
http://www.pcnineoneone.com/howto/cleandrive1.html
Seriously tho, if i remeber correctly, the DOD method is to randomly write a series of 1's and 0's across the whole drive 7 times, and then incinerate the drive with napalm or something. I forgot where i read that but, this is how they make sure that nothing can be recovered from the drive.
The normal way that I would clean my drive would be to just to do a format and reinstall of the OS. (so fdisk, format c:, then install os from the cd)