..and that's what this tool does. If you are going to sell your computer on e-bay, since you bought a new one, would you want any personal data to still be on the machine? This would be a great way of just eliminating that possibility.
This isn't really about just installing a new OS--I think it's more about protecting information on your obsolete machines.
Understood. I was just questioning the "I couldn't install an O/S on top of pre-existing data."
There are a handful of tools that do a good job of data wiping. I've used bcwipe with success.