Follow these steps to reset a password when there is only one administrator account on the computer, or if the original administrator account (of several) needs a password reset. "Original" administrator account refers to the one that was created immediately after installing Mac OS X. If the original administrator password is known, the original administrator user may reset the passwords of other administrator accounts using the steps described above.


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.

Source: docs.info.apple.com