Well, well, Ciara has a skilled badge. congrats
What you are looking for is software to do a zero format. Regular format is thought to erase everything and it does; unless you have special equipment etc.
If it were mine and I wanted to give my puter away, I would format twice, if possible.
If you are concerned,we need to look for zero format software.
Dban may be good http://www.dban.org/