I'm really confused as to why when you pin the icon to your start menu, it runs avast on the next startup. Pinning items to your start menu just keeps them on the start menu. It has nothing to do with programs that automatically run on startup.
Again, I feel like avast was somehow removed from the registry, and won't start up automatically anymore, and the easiest way that I can describe to fix that would be to uninstall, and re-install avast.
You could add it manually, but I don't want you to accidentally do the wrong thing and have your computer not boot at all again.