Windows is not activated

Hello. I bought a dell inspiron 15R SE 7520 with windows 8 pre-installed. Thereafter, I got the motherboard replaced twice while in warranty. Now, suddenly I could see that my windows 8 is not activated. When I click "Activate Windows", it opens PC settings and says that " Thanks, you're all done". But when I again open the system menu from control panel, it is still showing that windows is not activated. When I got the motherboard replaced, dell also provided me with new product keys. I also tried to key in both the product keys, but none are working. 

Another issue is with Windows Update. There are few updates that I am unable to install. After I restart the system for installing updates, it gives me an error saying " We could not complete your update".