Instructions
Press the "Guide" button on the Xbox controller. Press the "X" button sign out of your active account (if any).
Scroll to the "Home" tab.
Select "Recover Gamertag." You will be asked a series of questions (such as address, email and security questions you selected during registration) relevant to the Xbox Live account you're trying to recover. If you do not know enough of the information to pass the security check, you will be asked to contact Xbox Live customer support at 1-800-469-9269.