Yes, it is a known issue that firewalls block avast’s avastSvc.exe, which would block internet connectivity as it is blocking the web shield.

HTTPS shouldn’t fail as the web shield doesn’t monitor https secure encrypted traffic.

What is your firewall ?
Does it allow avastSvc.exe internet access ?

  • If it does delete the entry for it and reconnect to the internet, this should force the firewall to ask permission again.

If you have the XP firewall this shouldn’t be an issue as it has ZERO outbound firewall protection. So it is likely to be something else.

Have (or did) you another Anti-Virus installed in this system, if so what was it and how did you get rid of it ?