I've heard good things about DBAN. From about.com
Backup anything you want to keep. When the hard drive wipe is complete, there will be no way to get anything on the drive back.
Important: Sometimes multiple drives exist on a single hard drive. You can view the drives (volumes) that sit on a hard drive from the Disk Management tool in Windows.
Download a free data destruction program. I recommend DBAN because it's easy to use and can wipe a hard drive from outside of Windows.
However, if another program in the list suites you better, by all means use it instead.
Note: There are actually several ways to completely erase a hard drive but using data destruction software is the easiest and still allows the hard drive to be used again.
Burn the ISO file to a disc or complete whatever steps are necessary to install the software or get the program on a bootable disc.
Wipe the hard drive according to the program's instructions.
Note: Most data destruction program utilize several different methods to wipe a hard drive. If you're curious about the effectiveness or methods used to complete the hard drive wipe, see Data Sanitization Methods.
After properly wiping a hard drive, you can be confident that whatever information was on the drive is now gone for good.
You can now install Windows on the drive, create a new partition, sell or give away the hard drive, recycle or dispose of the hard drive, or whatever else you need to do.