Best to take it to someone who will "clean" the hard disk properly before it gets used again by a school or a charity.
www.digital-links.org is a fab organisation (NB - I have no other "interest" in them) which collects PCs from offices and cleans and recycles them for developing countries. I don't know if they will collect from home users, but they should be able to put you in touch with someone who will if they can't help
