Anleitung
Habs allerdings nie selbst probieren müssen.
http://www.macyourself.com/2009/08/03/how-to-reset-your-mac-os-x-password-without-an-installer-disc/
http://www.macyourself.com/2009/08/03/how-to-reset-your-mac-os-x-password-without-an-installer-disc/