I had the same problem on 32 bit XP pro some months ago, and avast tech tried to help me get it going. After awhile gave up and installed AVG.
Now I'm having the same problem on 64 bit XP (different computer) which is why I caught this thread.
There have been no problems on Vista 64bit, and laptop with XP home. I will reinstall and see if that helps.
I use a bastion firewall (untangle) with it's own DNS server. I can ping by URL name (i.e. google.com) and get response, but any browser hangs awaiting response from web server — apparently being blocked by avast web shield.