I recently had to uninstall AVG from both of my computers. After the most recent update, 90% of the internet applications stopped working. Firefox couldn't load pages and programs with automatic updates couldn't connect (including AVG itself.) It drove me absolutely crazy for a week before I figured it out. It probably took that long because, yeah, AVG is supposed to help, not sabotage your computer. It was the last thing I looked at.

Try uninstalling AVG and seeing if the problem goes away. If it doesn't, you can reinstall knowing you've ruled it out.