This maybe a little off-topic but hey always be sure: If you suddenly get a D/C or something and come in the main menu (it's where you come when you just started the game
) and you try to click on Battle.net and it says your CD key is already in use but you know you're the only one playing the game at the moment. What do you do then? It's quite easy: Just exit the game and go back in and tada, the problem fixed.