If your computer came with an install disk, boot to that disk and follow the instructions.
If if didn't come with install disks, there may be a recovery partition. You have to boot to that. If may be available by taping the F12 key on boot.
Either way, your personal data may not be completely erased. Maybe someone here has a program that will do that. This program states it will erase data:
http://sourceforge.net/projects/eraser/