First: UltimateBootCD.com. Run some of the diagnostics on it like memtest, mprime, drive tests. The first 2 should be run for 24 hours minimum. I've had brand new hardware fail some of these. I run these periodically to make sure my existing hardware is still behaving.
Second, Truly Wiping A Hard Disk: If you know a little linux and have a live CD, boot it, open a shell and run:
sudo dd if=/dev/zero of=/dev/sda bs=1024k
And wait a long time for the first disk to be low leveled. Change "sda" to "sdb" for the second disk, and so on. Once erased, you cannot get anything back. If your disk has special restore partitions and the like from over zealous manufacturers, that could cause you some problems. If you have retail windoze install discs, it shouldn't be an issue.
A couple of the disk test utilities on UBCD also have a drive wipe options. Not sure what they are, but I remember seeing them when exploring UBCD.
Quick edit warning: If you select the wrong disk to erase, YOUR DATA WILL BE GONE FOREVER, so be careful. If you have multiple disks and are unsure, open up your case and unplug every disk with data you want to keep.