Instructions
Turn on your television and Xbox 360, then log in to your Xbox Live account. By default, most accounts are set to automatically log in. A pop-up appears at the bottom of your TV screen letting you know if you were automatically signed in.
Navigate to the my "My Xbox" menu by pressing up or down on your Xbox 360 controller's left joystick.
Press the left joystick to the right to select your avatar. While your avatar is selected, press the "A" button to view your account.
Use the left joystick to select "Account Management," then press "A." From this menu, select "Manage Account" and press "A" again.
Navigate to the "Windows Live ID" heading using the left joystick, then press "A" to select it.
Select "Change Windows Live ID" using the left joystick, then hit the "A" button.
Press "A" to confirm "Yes" on the pop-up window.
Enter the Windows Live ID username and password that you want associated with your account. You can enter this information by navigating through the virtual keyboard that appears on your screen, or you can plug a USB keyboard directly into your Xbox 360 console.
Select "Sign In" using the joystick, and press "A."
Select "Yes, Change" when prompted on your screen, then push the "A" button.
Highlight the "Done" option and press "A" when you are finished. Your Xbox Live gamertag is now associated with the desired Windows Live ID.