I have an old xbox360 and my account is there. I want to download my account on the new black Xbox360. The thing is, I don't remember the email associated with that gamertag.
How can I get it?
Gamertag is still accessible on an Xbox 360
Go to your Xbox Guide (center button) => Settings => Account Management => Windows Live ID. The Windows Live ID associated with the signed-in Gamertag will show.
Gamertag is not on any Xbox 360
Contact Xbox support directly via support.xbox.com/contact. The site will give you an option to chat live or speak to a rep depending on the time of day. They are used to these requests. They'll ask you some questions to verify your ownership/identity before handing over your Windows Live ID.
Guessing your account details is most likely a waste of your time.
Go to the site and try emails which you think only the one which exists in the data base will be accepted if you remember your email addresses!