It might be a little over the top, but any time I need to reformat I use DBAN
it will completely wipe everything off your hard drive.
Where did you get your old computer, or more specifically your old OS. If you went out and bought your copy of Windows retail you shouldn't have any problems, but if you bought a premade machine from Dell/Gateway/HP... etc, the OS disc probably won't transfer over to the new machine and you'll have to buy a new copy of Windows... or come up with a good story to get a new product key. (Source