I would be inclined to call the Microsoft Activation phone number
and explain the problem to them.
It may be, unfortunately, that you have been sold a pirated copy of Vista. If so, and you can help Microsoft trace the person who sold it to you, they are normally very sympathetic and may legitamize your copy for free.
I'm assuming you didn't pirate the copy yourself, or download it, else you would be aware what the problem was!