I've had my eMachines laptop with Vista Home Basic (already pre-installed) for over 1 year. But yesterday all of a sudden I am getting a notice that says:: "This Copy of Windows is Not Genuine: build 6002"

The desktop wallpaper turns black all the time, and Windows thinks it isn't genuine! I bought this laptop (pre-installed with Vista Home Basic) at a large national retailer (PC World) so there is absolutely no chance this copy is fake.

I am being asked to "Activate Windows", and I have "0 days to Activate with a Product Key"

I really don't know what to do and why this is happening! Any help would be very much appreciated.
Thank you for your time