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 /

cd /var/db

rm .AppleSetupDome

reboot

On boot, the new mac wizard ran.  I was able to create a new account and remove the old one.