Instructions
Press the "guide" button on your Xbox 360 controller and press "X" to sign out of any alternate gamertag you may be using.
Press the "guide" button one more time and highlight "Recover Gamertag" right under the option "Create Profile." Another menu will pop up.
Select "Recover Gamertag" from the new menu.
Enter your Windows Live account info, including ID and password, and press "X" to continue.
Enter your personal information when prompted.
Select "Hard Drive" from the list of storage devices to save your gamertag and account info.