If you just install the OS on the hard drive im sure that will stop 99.9% of people from retrieving your files. Additionally what could you really have on a computer that old thats probably still relevant?
As an added thought that .01% that can do anything would have to target your machine to retrieve the files. Unless you have a particularly interesting past I would say just re-install the OS and you'll be fine.
Social security numbers remain relevant. I can't afford to take even a 0.01% risk with identity theft.
All of the foregoing comments notwithstanding, I gather that there is no completely effective way to erase a hard drive and, therefore, no completely safe way to dispose of one, short of destroying it. This is a real pain in the a**, given that we all have computers.