Instructions
Navigate to the "Games For Windows" page via your preferred web browser. Sign in to your Xbox LIVE account with your personal login credentials.
Scroll down the screen to the "Recover Gamertag" button. Double-click on the button.
Click on the "Recover Gamertag" button once more when prompted by Xbox LIVE.
Enter your Windows LIVE login credentials. Follow the on-screen instructions to complete the gamertag recovery process.