Instructions
Turn on your Xbox 360.
Press the "Guide" button on your Xbox 360 controller without selecting a profile.
Select "Recover Gamertag" from the "Home" tab.
Enter the Windows ID and password associated with the Gamertag you wish to move.
Press "A" on each screen and enter any information required. Recovering a Gamertag can take anywhere from two to 10 minutes, depending on the amount of information stored on your Gamertag.