Let me guess, the new web shield in Avast! blocks your HTTP port 80?
Strictly speeking, avast! WebShield does not block anything as it is not a firewall.
It just scan the HTTP traffic.
On contrary, the firewall could block WebShield access (and scanning of the HTTP traffic).
You should allow ashWebSv.exe to connect into your firewall settings.
Are you using a local proxy (annonimizer program, for instance)?