Build 7601

Hi I wonder if anyone can help. I had a new MB fitted to my laptop, it worked fine for 2 weeks after return. I am now getting the Build 7601 message, I have tried re-entering the product key but it tells me I cannot apply it to the computer. This is the product key this computer has always had..what can I do? Tried posting a question on the Microsoft forum but it just kept telling me to try...