It depends on whether or not the hard drive has to be in that old computer when you get rid of it. If it doesn't then physical destruction is the go. The easiest but rather smelly way is to open up the cover and take out the HD and then cook the thing in an oven or on the gas flame of a hotplate. Not kidding. Not much in the way of data is left after that. A magnet is a bit iffy. It might work but then again it might not wipe everything unless it's a pretty strong magnet. But a load of heat works wonders at messing up HD's. If you bash the thing a few times afterwards with a big hammer then there are very few people who would be able to do a lot with it. If on the other hand you have to leave the HD more or less untouched by such destructive methods then about all you can do is use software to do a complete "disk shred" with lots of passes. That will take ages but it's pretty effective. CCleaner is not bad (and it's free), but even then, I wouldn't guarantee that all the data will be 100% inaccessible to someone who has the right gadgetry to analyze it. EDIT to add: I see all the others who wrote in while I was writing my reply have said much the same things. We are not kidding. If you want to be 100% sure that a HD is well and truly unreadable, the only thing to do is treat it very badly with nasty methods like heat, hammers and so forth. Far better to literally smash, fry and mangle it than risk someone getting at your data, right?