Burn a live Linux DVD/CD. Boot it up. Login as root (administrator). Then, wipe the disc with the dd command. Assuming that this is the only HD on the system at the time, it will be /dev/sda. The command line would be this: dd if=/dev/zero of=/dev/sda bs=1M
This will write zeros to the drive in 1MB chunks. There may be Windows tools that can wipe a drive also, but this is the method I use to wipe client system discs when they want to sell/repurpose their systems. FWIW, this is NOT a forensic wipe - to do that, you need to write different data patterns to the disc at least 7 times. It is time-consuming, but effective.