It's a question many of us have asked ourselves (and some others should have asked themselves):
If a harddisk (or an entire computer) needs to be thrown away, how can you be absolutely sure no one will ever be able to get hold of your data that's stored inside?
Deleting it isn't enough. Deleting and formatting is a bit better. Deleting, formatting and overwriting is better still. But the method stoneshop  used yesterday is more fun, and even safer!