1. Shut down machine and unplug power supply
2. Fiddle with the RAM DIMMS. Specifically (carefully) remove a memory card and replace it. Be very careful that the static electricity from your person doesn't 'shock' the card. Wear an anti static strap or keep yourself grounded to the machine.
3. Once card replaced, restart machine holding down Opt+Control+P+R ('zap the PRam'). Hold keys down until you hear the Mac startup time three times. At this point the PRam has been wiped and you can relear keys. Password is wiped from memory.