I have the same issue on Windows 7 Professional 32bit.. It believe was also causing my xbox360 Media Center Extender to keep disconnecting after just a minute or two.
I've removed KB3004394 and I am now able to access Event Viewer and other items without it complaining about mmc.exe being from an unknown publisher, and the MCE appears to be working normally now as well.
Thanks for this post Doubletwist. I was having WMC extender issues too until I removed this update: