I hate to say, but this doesn't fix the problem.
The Action Center error message finally disappears, but that's after I clicked on "Turn on and update avast! Antivirus" (which seems to run something called VisthAux.exe)
Next time I reboot I'll do nothing and see whether the message disappears by itself (startup timing issue?)
(BTW how do I include screenshots in a post? "Insert image" seems to want a URL...)