Similar situation, with Windows XP. Downloaded new version of Avast 6.0.1367.0 and immediately lost web access (FF 8 and Chrome) and email (Thunderbird), although I could still ping and tracert. Also several modules of Avast were set to off, and pressing the Fix button or the individual buttons had no effect. Made sure the Windows Firewall was off, still no luck. Adding specific programs to allow had no effect. Got one few minute burst of access when I removed the battery from the laptop, but that ended quickly and never repeated.You should have tried a repair and reboot ;)
The only way I could connect was by uninstalling Avast.