I used to have a free program several reloads ago which would write randomly over the "unused" parts of the hard drive. Doing this several times was supposed to do what was, in effect, an irreversible erase of everything that had been deleted in the normal way.
with hard drives so cheap why bother selling your old drive why not use it for extra space on pc if people are worried about failed and broken drives and other people getting you infomatoin, then a HAMMER is a good answer.
Before we all get too security conscious ALL disk wipe programmes, ESPECIALLY the freebies, can be reversed by a forensic proggie that the Police use to nab the unwary. It costs shed loads o'cash but works very, very well.
However unless you are well shady then the wipe programme will be adequate to stop the spotty youth that has just bought your knackered 233 from revealing your 'Garden Tips for Coffin Dodgers' to the unsuspecting world.