“I suppose it hasn’t occurred to you that this is a virus, probably because you clicked on something while visiting a website.”
I’ve run comprehensive anti virus stuff on it...it is not a virus. Why would a virus take me to a MS website to buy Vista?
I don't know, and I'm unaware of anyone else having your problem.
When I have weird problems I search google for the message, to see if anyone else is having the same problem. If I had a valid MS copy of Windows and got messages saying it is invalid, I would go to my computer seller and ask why.
One small thing. I know for a fact that many Dell computers have Windows installed with a different licence number that what is displayed on the case sticker. There's a program called Keyfinder that will tell you what the real number is. There's a possibility that someone with a pirated copy has used your installation key.
In any case, after a day or so, I would consult google, the PC vendor and Microsoft, in that order.