Have you asked McAffe ? I run FSecure and they advised me to turn off defender, and to have just the one program running. When I did have both running never got any messages, just terribly slow... hogging resources and fighting each other. I would ask them... you've paid for the application... see what they say.
Dear mobileman1953 - thanks for your suggestion. My control panel doesn't have Services, but there is an icon for Windows Defender in the Security section. However on clicking on this I get the same "Application failed to start ..." error message that I see when booting up.