On April 9, Microsoft issued a Windows patch that 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.

After the update is installed, you won’t be able to log into Windows if you have affected antivirus software on your system. Windows will grind to a halt after you sign in.

This problem affects PCs with Sophos, Avira, Arcabit, Avast, and McAfee antivirus software. Microsoft has been continually adding antivirus programs to this list and McAfee is the latest one. To 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 did install the update before Microsoft put a block on it, you’ll likely need to install an update for your antivirus software to fix the problem. Microsoft provides more information about these known issues on its website.

It appears that Microsoft made a change to CSRSS—the Client Server Runtime Process— in this update. This change is causing problems with some antivirus software.

Of course, not all antivirus software is affected. We like Windows Defender and Malwarebytes. Neither is experiencing any problems with this update.