I Just installed OS10.4 on an eMac (1.25GHz/G4) and I lost my administrator privileges. The strange thing is, none of the accounts have administrator status (in preferences), so I can't simply change password using the startup disk. Also root was not active.
The first user on the system is usually the administrator. You cannot use or log in as root as user at OS X. However, if a system wide change is performed, you're asked to enter your administrator password.