miktex

I use both Win7 and Win8.1 on 2 different machines. This morning I upadated the MiKTeX packages in admin. Since, I have trouble compiling documents that used to be compilable as late as yesterday.
On Win8.1, the error message states: "This application failed to start because it could not find or load Qt platform plugin"windows". The recommandation message that follows is: reinstalling the application may fix the problem.
Which application is unclear.

On win7 (with all packages and fixes updated), the error message is: "The program can't start because QtGui4.dll is missing from your computer. Try reinstalling the program to fix this problem."
Again, it is not clear which application is required. I am guessing that it could be QuickTime, but as I never used this software, I unistalled it about 2 or 3 years ago (maybe even before).

I tried different documents but some do compile and others won't. They are all documents that I have compiled before.
Unhappily a MWE works on my Win7 PC but not on the Win8.1

How should I update this QT package?
Thank you
Regards.

You can fix this error by copying the file qwindows.dll from <install folder>/miktex/bin/platforms/ to <your user home>/AppData/Roaming/MikTex/<version number>/miktex/bin/platforms/.