Updated the other day and when I try to open Windows Defender I get a message that Windows Defender Is Turned Off By Group policy. When I reset the value of Windows Defender Disable Antispyware in regedit I get the error message: Application failed to initialize 0x800106ba.
Windows 10 is disabling Defender is a av is installed. (not sure if 8 is doing so already)
In Windows 7, avast disables defender when it is installed.
This is explained by someone from avast in another thread.