You can try to reinstall Windows, but that may be too risky. If everything is fine, I'm assuming that the problem is that the Windows doesn't recognize the RAM. That is the only explanation. If even that is ok, then you can assume that the RAM is defective. How many slots for RAM are there and how many sticks do you have, apart from your Corsair XMS3. First is, how many slots do you have on your motherboard? Have you tried swithcing places of your sticks? Since it's 4GB,a round number,i'm assuming it's the software that's faulty.