Security is not only having the latest patches installed (SP2), it is also not only related to a firewall or av software. The specific settings you make in them are also important. Also important is the way a user (you) handles the system and what (s)he does with it, what applications one installs, what sites one visits etc etc etc.

I'd like to know what the virus was.
Check it in Avast's log file.
What is a more worrying is the fact that I also caught another trojan (1stbar) when just accessing a site ie in a HTM file.
There are many variants with the malware family, so there are many ways they are trying to spread themselfs. Through mail, websites, applications, networks etc.

PS: I also answered the questions in your other thread. I think we overlooked that one. Sorry for that.