- Start from your Mac OS X 10.5 Install DVD. (You do not need to install Leopard again.)
- From the Utilities menu, choose Reset Password (do not choose Firmware Password Utility).
- Follow the onscreen instructions to reset the root password (the root account is not the same as your account). Specifically: Select the name of the drive that Mac OS X is installed on, select the user named "System Administrator (root)" from the pop-up menu, type the password in the first field, re-enter the password in the second field, then click the Save button.
- Restart from your Mac OS X Leopard volume.
- When the login window appears, select "Other..." and log in as the root user using the password that was created above. Note: If Mac OS X automatically logs in, choose Log Out (name) from the Apple menu to get to the login window.
- Go to Accounts preferences and check (enable) "Allow user to administer this computer" for the affected user.
- From the Apple menu, choose Log Out root....
- Log in as the user account that had this issue. (Your account should now have administrator access.)
- Use Directory Utility to disable the root user via Directory Utility's Edit menu. You may need to click the lock icon first).
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