Apparently Avast! is so good it’s finding virii that are not there.
Avast is lighting off a warning on certain work units from the World Community Grid project, a distributed computing volunteer project somewhat like SETI@Home used to be but with a more robust client.
I have put the entire WCG folder into the exceptions for Avast! and that appears to have stopped the problem, but it is something that I want to bring up to the programmers of Avast! as a possible issue.