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)
fdisk -l
  • 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.