I had this same problem start today out of nowhere. If I disable web shield I can browse as usual but can't browse with it on. I tried uninstalling and reinstalling but there's no change, all avast updates are installed.
Just before this started I was disconnected suddenly while online. I had to reset my modem and after that I couldn't browse with webshield on. Yesterday I installed XP SP3 but everything was working fine until today.