Dont have to format. Call M$ support line. Explain the situation to them. The should be able to give you adivce on how to fix this situation. If anything they will give you a ID # to activate this copy for now until you format again when you will be instructed to use your CD key.
Plus i think you can chage the CD Key. If you click on activate now does it give you the option to change your CD key? Or after it tries to activate does it give that option. If so then you can just put your CD key in and then try to activate.