1. Start up from a Mac OS X Install CD (one whose version is closest the the version of Mac OS X installed). You should first disable Open Firmware password protection, if it is enabled. Hold the C key as the computer starts.
2. Choose Reset Password from the Installer menu (or Utilities menu in Mac OS X 10.4 Tiger). Tip: If you don't see this menu or menu choice, you're probably not started from the CD yet.
3. Select your Mac OS X hard disk volume.
4. Set the user name of your original administrator account.
Important: Do not select "System Administrator (root)". This is actually a reference to the root user. Do not confuse it with a normal administrator account.
5. Enter a new password.
6. Click Save.