Considering that the last update version of avast 5.0 was 5.0.677 and avast 5.1 was 5.1.889 and now we are on avast 6.0.1000, I don't know if there would be any point in chasing this down in a very old version of avast.
I can't see any reason why avast wants to start adobe reader, I have never seen it and I have been using avast for almost seven years. Avast doesn't require adobe reader to function. I stopped using adobe many years ago (for FoxItPDF reader) and no complaints from avast about not having it installed.