If your data is not classified US Govt Top Secret, or better, then just delete it.
Dump your recycle bin and do all the normal hard drive cleanup stuff.
When you say OLD COMPUTER, just how old are you talking about?
Windows 98 or ME old?
If so, download and run "Hoover.bat" which you can get for free from Here.
It will dump all the temporary, history and cookies folders and leave that PC in a much cleaner condition.
Delete all your old data files and then run Scandisk and Defrag.
Defrag will move files around, overwriting all the stuff you deleted.
If you're into reloading everything from the OS to all the hardware drivers onto that old PC, then by all means go the "KillDisk" route.
Or just get OnTrack's "EraserHD.exe" from Here.
The downloaded file will build a boot floppy for you. Then just boot up with it and follow the on-screen prompts.
You'll have a pristinely clean HD when it's done.
Good luck and happy computing,
Edited by The Shadow, 27 May 2006 - 10:49 AM.