kiwi1691:lapimate: Has updates (Dec 2014) installed KB3004394? This is one of the issues sometimes mentioned in respect of that update: http://help.howproblemsolution.com/922574/windows-suddenly-not-genuine-kb3004394
KB3004394 has been withdrawn. See http://support.microsoft.com/kb/3024777
I downloaded the update, but when I try to open it nothing happens.
Right, that looks somewhat legit.
However with the above You mis-understood.
The idea is to NOT install this patch, as it causes the problem. If it does nothing its probably already there.
Look in add/remove programs. Check the show updates and remove that one from the list
http://www.infoworld.com/article/2859267/operating-systems/windows-7-hit-by-rash-of-bogus-not-genuine-reports-validation-code-0x8004fe21.html
Other things that cause it is change of HW on OEM copies. Not made any changes?
You can run the diagtool and see if it reports why after removing the above patch if it exists.
MGADiag tool