Dban

Dban is a utility that permits you to securely wipe the contents of entire hard drives. You can use it to wipe data off the corporate laptop that you are returning to your employer, a hard drive that you plan to dispose off or your hard drive that you plan to reinstall the OS on because it is infected with malware. You can download the CD, DVD, diskette or USB flash drive versions for whatever your needs.
Boot up using Dban. When you see the menu, type in "autonuke" at the boot prompt. This is the easiest mode to use. Once you've pressed "Enter" key, Dban goes about its business and does not require any further interaction from the user. Viola! Simple as ABC. Patience is required as wiping is not a speedy process. Time taken to wipe a whole drive varies depending on the wiping method selected and the size of the drive.

For advanced users, you may opt for other wiping methods by pressing on "F3". The DoD 5220.22-M method and the Gutmann* methods are quite time consuming.

Active computer forensic guru conducted independent research and arrives at the conclusion that a single drive wipe makes data impossible to recover.

* From personal experience, Dban took 2 days to wipe my 40GB hard drive using the Gutmann method.