Mike is correct, up to a point.  Just a few screws to take the drive apart, and you have some very good strong magnets to play with or use in a project.

I would never sell a computer with a drive I have used, plus the fact that drives are very cheap these days. 

I have been told that after wiping the disk, making several passes nearby with a strong magnet will assure no recovery, but I wouldn't trust that.  Depending on what you think of your data, and if you would ever want someone else to see it, make yhour own decision.  Chances are, nobody will really care about anything you have on yhour computer, and never try to retreive it. 

But, if you are  a convicted felon and have proof of that on your computer, by all means destroy the drive.