Hiya,
I've got the same problem apart from when I authorised my computer and tried to download my music and such, it comes up with a message:
This computer is already associated with an Apple ID.
You can download past purchases on this computer with just one Apple ID every 90 days. You cannot associate this computer with a different Apple ID for 30 days.
I have never changed the Apple ID. This is a new laptop and I am trying to get all my old music and films back onto it. I have waited a few days and the "30 days" part of the popup has not reduced to any number. I have tried contacting Apple but because I live in the UK there is no support service available. I have paid for all my music and I would preferbly like to listen to it :/