On April 9, Microsoft released a Windows patch that broke PCs with specific antivirus programs. This affects PCs running Windows 7, 8.1
After installing the update, you will not be able to log into Windows if you have done this affected antivirus software on your computer System. Windows will come to a standstill after login.
This issue affects PCs running Sophos, Avira, Arcabit, Avast, and McAfee antivirus software. Microsoft has been continually adding antivirus programs to this list, and McAfee is the latest. To help protect users, Microsoft has added a block to this update that prevents it from being installed on PCs with affected antivirus software.
If your PC has installed the update before Microsoft has blocked it, you probably need to install an update to your antivirus software to resolve the issue. Microsoft provides more information about these known issues on its Web site.
It appears that Microsoft has changed CSRSS – the client server runtime process – in this update. This change causes problems with antivirus software.
Of course, not all antivirus software is affected. We like Windows Defender (which is Microsoft Security Essentials on Windows 7) and Malwarebytes. There are also no problems with this update.
Many thanks to Ars Technica and PCWorld for lighting this spotlight.