If you really really REALLY need to completely wipe out the data, google Darik's Boot and Nuke. It goes on a bootable floppy/cd/flash drive; you boot your computer from it, pick options and it proceeds to wipe your HDD to government standards.
You'll be looking at several hours of your computer running around 100% CPU, but they guarantee there'll be nothing left. I've done this with a few computers I've donated.
EDIT: dban.sourceforge.net is the URL for it.
thanks every one
i already had this one on a floppy but when i put xp on it i got swamped with viruses
i thought it loaded 'em, i guess i needed to get a virus blocker off a cd next time
once again thanks