The last major Windows update broke PCs with certain antivirus programs installed. This affects PCs running Windows 7, 8.1, Server 2008 R2, Server 2012, and Server 2012 R2 —not windows 10, this time.
Once the update is installed the affected machines seem fine until prompted to log in, at which point you won’t be able to log into Windows. Some users have reported that they can log in, but the process takes ten or more hours.
This problem affects PCs with Sophos, Avira, ArcaBit, Avast, and McAfee antivirus software. Mircosoft is currently blocking the update for these users. Some antivirus vendors have published updates for the problem.
Mircosoft provides more information about these known issues on its website.
Not all antivirus software is affected. Windows Defender (Mircosoft Security Essentials —windows 7) and Malwarebytes are not experiencing any problems with this update.
We highly recommend if you do have the affected antivirus on your systems to change them now.