The best - and certainly the most enjoyable - way of securely erasing the hard disk is to remove it from the PC and beat it to a pulp with a club hammer.
Vent the frustrations the PC has caused you over the years in a frenzied assault on the hard disk.
Of course, it is not possible to re-format it afterwards, at least not in the conventional sense! And no defragmenter will be able to defragment it ever again.
For the more technically curious, see how much you can open it up and dismantle it before the percussive formatting mentioned above.
SD
Oh - almost forgot, your local council tip probably has an area for electrical / electronic stuff that you can leave it (sans hard disk).