In my case the latest Avast update broke internet connectivity. After the update, I could still connect to IRC and use various instant messengers, and https addresses would work, but regular http would not. Disabling Avast finally made the problems go away.
Win 7 Pro 64-bit, also running ZoneAlarm 10.1.065.000 - and removing ZA is not an option. If there's an unresolved conflict, Avast is the one getting the boot.