General Questions

I have a Monowall routing 3 LAN networks with ~20 computers to the Internet. I simply want to log Internet traffic per machine / IP address, eg I want to have a graph or log file showing per hour / day / month Internet / WAN ONLY usage of each machine.
All servers are Windows based, I don't know or use linux.
What is the simplest way to do this? SNMP? Syslog? Nothing I try seems to do this or is wildy over complicated. 

I would use captive portal and RADIUS to do accounting.  Then run reports off the RADIUS database.  IF you want, you could even set the redirect page to the report for that user so they see their use each time the log in.

Thanks Lee, I may try this in future if the bandwidth usage keeps being an issue. For now I have found that you can run ntop on windows, and have set that up with a managed switch to find the information I needed, and found the source of our Internet overuse. This is very much better than any of the other solutions I had tried.