You can take the unit to any apple store... make a Genius BAr Appt and ask them to please reset the PW on the machine. They have the disks (either DVD or HD version) you need to do that. If you want it Erased and reinstalled, they might even do that while you wait... ask nicely

. If you want to have something to use at home... just buy the highest allowable OS that can go on the system... many G4's and G5s run Leopard and all intels run snow leopard (10.6) just be sure you meet the minimum system requirements (HArd Drive space, RAM, etc.)
Good Luck!