To keep those secrets safe, beat your computer's brains out with a hammer. "HAVE YOU been reduced to a state of such furious impotence by your computer that you wanted to smash it into a thousand tiny pieces? Perhaps the world would be a safer place if you did. Businesses and private users are failing properly to delete sensitive information stored on their computers before the machines are sold on. This has made the burgeoning trade in secondhand computer parts a potential treasure trove for blackmailers, paedophiles and fraudsters . . ."
Another article on used hard drives . .Clean the Hard Drive Before Dumping Your PC.