Instructions
Press the "Guide" button on your controller.
Select the "Recover Gamertag" option and then click the "A" button. This loads a confirmation dialog.
Press the "A" button to confirm. This will prompt you for your security information.
Enter your Xbox Live Gamertag and select "Continue."
Enter your Windows Live ID and password and select "Continue."
Select "Sign In" and press the "A" button. This loads a "Terms of Service" screen. Select "Accept" and press the "A" button. Your profile and gamerscore automatically downloads onto your Xbox 360.