Problem: Threatfire

Solution: run services.msc as domain level admin, set startup type for Threatfire to disabled. Exit Threatfire (CTRL + RIGHT CLICK → EXIT). msconfig → Startup → untick Threatfire (couldn’t be bothered with GUI for now).

Then reboot, unblock Avast setup file, install.

Reboot, then wait for Avast to run. Re-enable Threatfire service, start threatfire service, start threatfire. Both work fine again side by side as they have done for YEARS. The only hint (to a novice) that Threatfire was the problem was 1 threat detected at the end of the install. I permanently allowed the process to continue, and thought nothing more of it. Seems that was the killer…

PERMANENT WORKAROUND:

Set the startup type for ThreatFire as Automatic (Delayed), and make sure Avast service is set to Automatic. Then it will allow Avast to run first, and Threatfire will quite happily startup later (about 5s depending on your setup) and run as well.