Use the Utility that is (most likely) built into your hard drive.
According to the Center for Magnetic Recording Research (CMRR) at the University of California, San Diego, every disk drive manufactured since 2001 and over 15Gb in size has a built-in wipe (erase) function that has been approved by the U.S. National Institute for Standards and Technology (NIST), Computer Security Center.
This function is built into the drive, and it will erase ALL the sectors on the platter regardless of how the disk is formatted (unformatted areas are wiped), and regardless of whether given sectors have been marked as bad (they are wiped also)
CMRR provide a free utility that invokes the function and which should be run from a native DOS boot.
Their page at http://cmrr.ucsd.edu/people/Hughes/SecureErase.shtml includes
- A disk wipe tutorial (this is worth reading even if you do not need to wipe a disk right now)
- Download of the erase utility
- The Read Me doc
- A FAQ