I set up a brand new XBox Live account for me and my son. When I then went onto our XBox 360 machine to sign in and use the service, it didnt let me sign in. The 360 only gave me an option to sign up for an account, not sign into existing account. When I entered either mine or my sons account info, it rejected ti saying it was already associated with another Xbox Live account. Please help us sign in and use for first time!!