First thing you will need to do is remember when you bought your computer. If you bought it before 2001 then skip to Step 6. If after 2001, then continue reading... If you aren't sure you can try either method, but make sure you have 2 cds incase one doesn't work (then you can try the other)
There are several tools you can use to 'nuke' your drive, but the best is called 'HDD Erase'. It is best because it is simple software, that triggers a feature built into the firmware of your hard drive (if built around 2001 and later) that destroys everything. You can read about it here http://cmrr.ucsd.edu/people/Hughes/SecureErase.shtml
. I have tried to use the file they supply to make a cd, but it didn't work, so I'll show you another way.