Several years ago I bought a truly legitimate key from a reputable online vendor and the OS ran just fine for a couple of years. Then suddenly the program "deactivated" itself. I called Microsoft and they told me the key had been compromised and it had subsequently been blacklisted. They would confirm that the key had been genuine at one time (aren't they all) but was no longer going to be honored. I had no recourse but to either buy another key or find another solution. I usually opt to purchase genuine, but in this case where I bought a Microsoft product in good faith, only to have that product taken from me arbitrarily, I chose not to buy another genuine. Hence my discovery of this fabulous forum. I believe there are certain universal truths: if it seems too good to be true, it's usually is and (in the case of Microsoft product keys) what's genuine today, may be be blacklisted tomorrow.
Instead of mentioning MSFT as much you should mention the socalled "reputable" online vendor You most likely have bought a key which wasn't meant for selling.