Find out the manufacturer of the hard drive (maxtor, seagate or so on). Go to their website. They should have a download section for utilities. One of them should be a reformat tool.
You can follow the instructions that will probably have you install the program on a CD or a Floppy.
You load the cd or floppy. Restart your computer and it should load up the program (if your computer it set to check the floppy or cd drives first).
Another way....
If you have say a windowsXP cd. Put the CD in, Restart your computer. Let it load the windows setup and choose the option to install a fresh copy of windows and follow the instructions to reformat the hard drive.
Another way...
If you have another computer. You can hook up the hard drive you want to wipe as a slave or secondary drive. (make sure of course to turn your computer and remove the power cable while you are hooking up the second hard drive, also make sure to use the correct jumper settings on the second hard drive). Restart your computer. Windows should load and it will recognize the second hard drive.
This should add an extra letter (drive) when you click into my computer. So if you used to see A, C you might now see A, C, D.
Find the letter for that second hard drive. Right Click it and choose the FORMAT option from the drop down menu...follow the instructions.
Hope that helps.