DavidR
14
You never answered the question about your firewall and this is the most common rout of entry as there could be an undetected trojan downloader that connects to the internet to download this brontok and possibly other stuff, when it arrives avast can detect it but is unable to stop it as what may be downloading it would appear to be just another file with no payload.
That is why we suggest the inclusion of anti-spyware tools which are more likely to find the trojan downloader. However, your firewall should also be playing a part in protecting against unauthorised outbound Internet Connections.