Mac OS X requires a password to make changes to the system, when starting some programs, such as Terminal, or when unlocking the computer when it has been configured to lock when it's booted. If you have forgotten your account password or purchased a MacBook without knowing the main account password, you will not be able to log in to it. You can change the main account password and log in to the computer with the OS X install disc.
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Insert the Mac OS X install disc in the MacBook and restart the computer. Hold the "C" key while the computer starts to force it to start from the disc.
Choose your language and click the "Arrow" icon. Click "Utilities" and select "Reset Password."
Click the hard drive in the window to select it. Click the username for the main account in the drop-down menu. Type a new password, then re-enter it in the confirmation text box. Type a password hint for the account. Click "Save."
Click "OK" on the confirmation pop-up window. Click "Installer," then "Quit Mac OS X Installer." Click "Restart" to reboot the MacBook.