Aha - never mind, got it. It was not saving it. Running the configuration as adminsitrator did it

Just got back from buying this, installed it and tried entering the serial key. But when I start the game it says it is invalid. I've tried all manner of ways of entering it, capaitalised, non capaitalised, with the dashes, without the dashes, spaces, etc but it won't accept it. When I restart the c...