create a new account on xbox and try starting with the new account.
if it fail again, try this steps:
Logout from your account
start cod MW2
now the game should start and ask for a gamer account. now log in and play the game.
if the 2nd advice is working for you, let me know. i'll guid you. i hade the same damn problems with COD MW2, COD BO 1 and it was very frustrating. I was not able to start the original Game disc or from harddrive if i was logged with my gamer account. all the games only worked, if i was "logged out" before i started the game.
so long