I would think that the problem is caused by some of WMP's default settings being changed by something at some point. I don't think Avast! is the culprit there because if it was , everybody with WMP11 on XP would have the problem and only a minority do. The Fixit tool most likely restores all of the default WMP settings and/or scripts. They may have been changed by updates from Microsoft (but again, you'd think everybody would have the problem is that was the cause) or by other things but obviously something was altered in the scripting somewhere along the line and the change doesn't mesh with Avast's Script shield.
I agree that somebody should step up and allow the dev's access to try to find and eliminate the problem.