You can wipe a harddrive easily using Finnix - the LiveCD for sysadmins:
- Download and burn the CD
- Boot the CD
- List the drives (the names of the disks are the ones that start with /dev)
- Wipe the disk (in this case /dev/hda) and give a beep when it's done
wipe -kqQ1 /dev/hda; echo -en "\a"
Warning: This will wipe the disk unrecoverably - which of course is the whole point - but wiping the wrong disk can be quite bad.