There is a program called KillDisk which is free just google it. It will completely erase the HD, you will need OS cd to reinstall your OS.. KillDisk does not only erase the hard drive it makes it so the information cannot ever be retrieved, which formating does not do, so I heard.
A low level format writes zeros across the entire surface. You will then need to create partitions and then format (standard high level) the partitions with the FDISK function. You will also need a full version of your OS not just a recovery or upgrade disk.