If you did not say that you bought your DVD from a reputable firm, I would suggest that it is indeed a counterfeit copy. If you did not notice any problems with the packaging, torn wrapping, key code that did not look right etc., we should, for now, assume that you are legit. If that is the case for whatever reason your machine is malfunctioning and indicating that you do indeed have a counterfeit copy. I am not really familiar with a situation like this, but I would assume after the 30 day period has expired, the MS computers will pick up on the notice. Then the fun begins. They will periodically shut you down, not send updates etc. You do not want to get to that point. Take your time, but if you cannot ascertain a valid reason for this problem, I think it wise to call MS, explain the problem to them and hope for the best. Worst case scenario, they will confirm that your number is not legit, and yes, you have to buy another. A more likely scenario, is that they will confirm that you are legit, make note of that fact and set their computers to disregard the bogus reports that they get from you, they may even give you another number which will work correctly.
Regarding the call to MS, I would call the number, used when you cannot activate from your computer. This alone may clear up your problem. If not you will speak to someone and you should follow the guidelines that I provided. Anyway good luck. Its a shame, after spending good money, you must get aggrevated.