Hey...
One thing you can do to lessen traffic, although it isn't a great amount, is to block "Host Process For Windows Services" from network access. This is needed for Windows Update to work and when you do so, you can manually allow it then when done, set it to block again.
What you really need to do in this scenario, if you are really that serious on finding all traffic and logging it, is to install Wireshark on every machine. Every little packet that is sent or received, across the lan or the internet, will be logged. You can then easily apply all sorts of filters to the logs, such as packet type, destination (lan, wan or other types such as ICMP, Syn/Ack, UDP, TCP etc...).
Once you have a look at these things on all machines...