Fantastic resource first of all!

My question relates to me trying to understand all the different things I have been told about Net security.

I am of the understanding that a firewall closes all your ports and asks you if and when any programs attempt to access the web from your machine or if any machines attempt connections to your PC from cyberspace.

Now my question is if you keep upto date with all the security patches and updates etc. As long as you do not have any areas to exploit and as long as you do not download or open up any trojans, whats the point of having a firewall?

My understanding is that even if you have an open port the only way anyone can contect to your PC via the open port is if there is a server listening to that port from your machine. Hence if you make sure each time you connect that nothing is listening to the ports that should not be by using netstat -a -n (I think it is) there is no danger.

Please correct me if I have become totally confused in all of this.