Assuming that you can still boot the PC there are a variety of programmes that will securely erase the hard disk. I use
Active@ Killdisk but there are others too. Don't forget to take cd/dvds out of the cd/dvd drive too! There are some organistions that refurb old PCs for use in Africa etc it might find a useful home in one of these.
ComputerAid are one of the organistions that do this