Seeing you're using Internet security my bet is on the Firewall. See
my thread about the issue.
A few things you can try:
- Turn off windows firewall if it's somehow still active and conflicting with Avast's firewall
- Repair the Avast installation (via (un-)installer) or remove -> clean -> re-install
- Last resort: Remove the Avast IS firewall and use the Windows firewall (see instructions in the linked thread)