activation problem

I puchased a windows upgrade for my two home computers (laptop and desktop) from Costco a few months ago. It had the 64 bit version which I needed for my laptop and the 32bit version for my desktop. The laptop was running Vista and the desktop XP . I upgraded both machines and all was well until yesterday. Now after an update, my desktop says the windows version is not genuine. I retyped the verification code, but it will not take it. I'm assuming this is because I used it on 2 computers? Do I have to pay for another?

If you sued the same key on both machines then yes you need to buy a licence for the second machine. You can then phone microsoft and get the other machine activated.