i'm having the exact same problem in vista 64 bit.
internet works in messengers and other programs, but all browsers are blocked.
that's IE8, Firefox, safari, Google Chrome, Opera.
happend a few hours ago, everything was working fine upuntill then. i've uninstalled AVG free edition now though, hopefully can find a fix.
i'm sure that there are many other people who have the same problem, but they don't know what is causing it, or how to ask for help, alot of people panicking out there. like i was and had to use the steam games browser.