Ok I did what Muad'Dib said and it seemed to work! The first time I played however, I was disconnected from the internet. But today I played for about a hour (And usually before, the Avast pop up would show up sooner than that) connected to the internet and it still didn't pop up.
Thanks for your help guys! And thanks for finding how to disable the Avast pop up while in full screen Muad'Dib!