Instructions
Turn on your Xbox 360.
Press the "Guide" button on your controller. Select "Sign In." Select the profile (gamertag) you wish to sign in under and press "A."
Press the "Guide" button. Press the joystick to the right twice to reach the "Settings" window. Select "Account Management." Press "A."
Select "Windows Live ID" and press the "A" button. The email linked to your gamertag will be displayed under the "Windows Live ID" window on the right-hand side of the screen.