I just had this happen right now. Avast updated when I updated my PC, and the browser just installed itself and set itself as the default. There was no window, no checkbox, no warning, I didn't even click any kind of "update notification"... I just updated my PC and Avast did this all by itself.
I am furious, and feel violated... this decision was taken out of my hands entirely, and now I have to clean up after Avast.