Spiceworks would work, and is fairly straight forward to set up IIRC, but it is serious overkill for what you want. Just install it on one PC and make sure it can scan and find the others.
http://www.spiceworks.com/
You could even do something as simple as run the "netstat -e" command and record the data already transferred and do the same the next day. Not foolproof though, as the computer will need to be on the whole time.
I think the best solution though would be flashing something to your router that would allow you to monitor it at that point, but I am not sure if that will identify specific IP addresses within your network.
Something like
http://www.dd-wrt.com/site/index or
http://www.polarcloud.com/tomato