Well, followed Forejt's instructions and, after a lot of effort (for some reason, when I've started in Safe Mode the apparently normal restart doesn't load Avast and Comodo services, so I have to use F5 and go to last known good), Avast is up, running and showing 4.8.1169.
Actually, due to having conflicting information on version from 3 sources, I'm wondering if the version of the .exe files was munged as well. I was puzzled by the version here and carefully checked that it was .7 not .8.
Thanks to all for advice - bit happier now.