I recently picked up a Powermac G4, running Mac OS X 10.3, and it had an account on it with unknown credentials. I needed to reset the account.
Boot into single user mode (reboot, at the grey Apple screen press Command-S (or Windows-S))
I typed the commands:
Mount –uw /
On boot, the new mac wizard ran. I was able to create a new account and remove the old one.