Did you try booting from your original Installation CD? If you are running 10.4 pop in the CD (which is really a DVD!) boot with the letter "C" held down until you see the apple logo. It will launch the Installer, but be not afraid. Choose your language, click on Utilities in the menu bar and go down to reset password. One of your choices will be "root". I am curious, how did you manage to let the machine accept no password?!