If you know who makes the hard drive you can go to the manufacturers website and download their utility programs. This will be free, and as far as I know all of them have a program that will do exactly what you want quit well. What you want to do is format the drive. Then write all 0's to it to wipe out any last spec of data might be left.
It is really pretty easy to do, and will come with instructions. Generally though what you need to do is stick the program on a 3.5" floppy disk, this disk will become a bootable disk. Then reboot the computer with the disk in the drive, and it will give you all the options of what you can do. First let it format the drive, then choose to 0 fill it.
Or you could follow Chuck's recommendation which is a great idea. You get some extra HD space, easily keep any files you want, and can be absolutely certain that no sensitive information got to the new owner.