Hi:
No big deal, just tell M$ that you started your machine and it want to be reactivated again. They will ask you for the installation codes (the long string of numbers only) and give you a new activation code.
This has to be some sort of an activation bug issue with Vista as mine had done the same thing, but I do not use vista near as much as XP. I find vista to be nothing more than a PITA with window dressing.
MV