I have been using avast since January along with Windows XP and i had just experienced the same problem which made me reinstall my windows and avast last December.
After an update, my web browsers are all blocked and the only way that i can browse the web is by use of the windows explorer (typing http://websiteurl at the address bar). All other applications that need internet connections are working properly like my IM yahoo messenger, my online games and everything else not labeled as WEB BROWSER.
I am only using windows firewall and avast running on windows XP SP2. The only solution that I do so far is to stop all avast services except the standard shield. I have tried putting avast,firefox in the exceptions of windows firewall but still to no avail. I can only browse with the use of windows explorer.
This problem has occured to me like 4 times already since last year and all i did those times was to uninstall avast and use AVG(same as reformatting since i will catch a virus later on) or reformat. It seems like the problem occurs when my avast updates. Can someone help me?