I have had the same problem (and just lost 3 days of work troubleshooting and performing system restore operations). The symptoms are: system fails to reboot/shutdown, some software and Windows services (e.g. Task Manager) fail to load, and Avast fails to appear in system tray. System will hang doing basic operations, and is basically rendered useless.
As far as I can tell Avast (I have the free version) is definitely involved in a conflict with KB3000850, but I also installed Spybot 2.4 and the latest version of Trusteer Rapport Endpoint Protection and I believe it may be some combination of these software packages running under this particular Windows Update. I was running all of these software packages prior to the update with no problems. However, once I installed the update the system had major issues. I was able to run Avast with the update by itself but the problem cropped up when I installed Spybot. I uninstalled Avast in Safe Mode and the problem went away.
A tech support rep from Avast told me the problem was probably the result of running multiple anti-malware programs (i.e. Avast free and Spybot) but the free version of Avast is not a full featured anti-malware program and it's common to run anti-malware software (e.g. Windows Defender or something else) alongside it.