If you have a copy of the OS install disks, then I'd suggest wiping the drive clean and then putting a fresh OS install back on the system.
If you don't have a copy of the OS install disks, then you should see if there's a recovery partition on the laptop as seen suggested, and put it back to factory specs, which will wipe out the drive.
Realistically, it depends on what you want to wipe off the system. If you've had it a while, then doing a fresh OS reinstall would be the cleanest way to do it.