Thanks for the tip! For now, I think I'm going to leave things as they are. Avast 5.0.677, which I now have installed, has worked fine with OA for several months. In fact, Avast and OA have co-existed happily on my computer for 3-4 years. It was only when trying to install 5.1.x that a problem occurred. But now I know where to make the exclusion of OA, if I decide to do that.