What do I do about "not genuine" Windows?

I have an Acer Aspire laptop, bought in 2011. Some time yesterday afternoon my wallpaper vanished and was replaced by a black screen, on which were all my icons, with their names (nothing that hadn’t been there all along), and in the lower right a little message in white letters that said “This copy of Windows is not genuine.” Windows Activation says “Status Not Available” and “Product ID Not Available.” I contacted Microsoft, and the first tech I worked with helped me replace the black screen with buff, but the message is still there. The third tech I got said that my product key was some kind of mass license and they had no warranty for it, but Acer might. Apparently that's not true, but I still want to get rid of the “not genuine” message and get genuine Windows back, because I had a message that said the “not genuine” one might make me vulnerable to worms, etc.  So how can I do this, ideally without having to buy a whole new Windows suite?