1. Insert the Mac OS X Install disc and restart the computer. 2. When you hear the startup tone, hold down the C key until you see the spinning gear. 3. When the Installer appears, choose Utilities > Reset Password. 4. Follow the onscreen instructions to change the password. 5. Quit the Installer and restart your computer while holding down the mouse button to eject the disc.
You can also call apple & they can guide you on what to do as well.