I use DBAN myself , when i sell a computer.
How to Use DBAN
Darik's Boot and Nuke (DBAN) is a self-contained boot disk that securely wipes the hard disks of most computers. DBAN automatically and completely deletes the contents of any hard disk that it can detect, which makes it an appropriate utility for bulk or emergency data destruction.
DBAN's website is http://www.dban.org/.
How to Use DBAN to Securely Erase a Hard Disk Drive
Turn on computer and boot from the DBAN CD.
The DBAN application opens. DBAN is a simple, text-only application.
You will see a boot: prompt and a menu of options.
Press the Enter key to start DBAN in interactive mode.
DBAN continues to boot. This will take a few moments.
You will see a screen listing Options, Statistics, and Disks and Partitions.
Leave the defaults for Options.
From the Disks and Partitions list, select the hard drive you want deleted.
Move the cursor up and down using the J and K keys, and select the drive with the Spacebar.
When the drive you want to erase is selected, press F10 to start the wipe.
The time required to wipe your drive will depend on the size of the drive. DBAN offers an estimate on the Statistics pane.
http://www.it.cornell.edu/security/depth/practices/dban.cfm
Disk will need to be formatted after using this process.