0x20000004 error is a “proxy down” error, i.e., server unreachable (or, in other words, “cannot connect to the updating server”). Is there any other Firewall installed on the machine besides Windows own firewall? Have you had any previous anti-virus program installed on this computer before installing avast? The last way is check your hosts file to see if there are any entries that could be blocking avast’s update servers. A blank “default” configuration of the windows hosts file should basically have only one entry (127.0.0.1 localhost).