If you have a Windows CD, you can put that in, reboot the computer, and boot into the CD (you may have to adjust the BIOS setting for what boots up first, which you can do by pressing F2 at startup and looking for a setting for startup options...you'll want CD listed before HDD or Hard Drive).
Once you do that, you'll be able to reinstall Windows, which will wipe the hard drive in the process. Then just login (you'll choose the login when you install) and make sure there isn't anything leftover (on another drive, perhaps, though with a laptop that's unlikely) and you're good to go.
