Okay, like many users I had downloaded the latest (RTM) version of the Windows 8.1 Update 1 and installed it as described in instructions (with all proper restarts after each update, I did not need first update of the four, as I had the March patch installed already). All was fine until I installed the last tiny update - Feature Pack for Windows (KB2937592). The update told me that it needed a reboot (though Wzor description was telling it did not need it). After reboot and logging into the account, my screen went completely blank, it stayed black for a while, until strange start button started appearing in some places on the screen, then all went dark, PC rebooted and then was all normal. Update is listed as successfully installed, so says the Event manager. sfc /scannow showed no corruption. dism also did not need to repair anything. About my Win 8.1 installation: it was Win 8.1 SL 64-bit RTM which later I upgraded to Win 8.1 ProWMC. Only default English language installed. I had all updates installed from Windows Update, including the March patch Tuesday. All GA Rollup updates were installed through WU. I had never used .msi packages on this system before, it was completely clean and nice install. Any guesses? If this is related to the fact that I had SL edition upgraded to ProWMC and this is a bug, then I guess microsoft did screw up with this last tiny feature pack update.