Insert the installer disc 1 (grey original disc), and start with the C key held down after the chime. When the installer appears, OK the language page, then ignore the installer and go to the menu bar at the top of the screen.
I think it's the Utilities menu, but you can try the others if it isn't - select the password reset (might be password manager) option and follow the instructions.
Then quit the installer and reboot normally.
(That's for all OSs up to Snow Leopard - if you're on Lion, someone else will need to help you)
Earlier than Lion: