don't use a magnet yet. install eraser on your pc. right click on your data and click erase. click options. click guttman and click ok. click ok/erase again to erase. after erase, open eraser in your start menu. click file>new task. then click ok. click edit>preferences>erasing. click unused disk space. click on guttman and click ok. click on task menu>run tasks.
Wait wait wait for 1 hour until finished.
Then, unplug your PC, pull out the hard drive. if you have got a screwdriver (those with the kind of plastic handle) and unscrew the hd. have some googles on your eyes. it might blow up. Some air should rush into it. the hard disk needs to be in vacumm, it won't work now. use a magnet. break the disk into two. throw it away. done!
-0- PS. if it doesn't work, don't find me. A friend of mine unscrew a mac harddisk and it broke up. i am not sure if it will blow up, take this at your own risk. or, just use some spray on the hard disk (bug spray) and throw it away. "throw!" it will blow up if there is a current, since spray is made of oil.