Tracking Network Traffic to Workstation

I need either a software or hardware solution to monitor our internet traffic. We have a full T1. We use a Watchguard Firebox 700 for our firewalling and NAT.

The management software for the Watchguard has a bandwidth usage meter, which is often slammed to the top. There aren’t any other effective tools to see who is doing what. I need some way to figure out what is using up all the bandwidth!

I would like to trace this back to the offending workstation but do not want to have to install software on each workstation to achieve this.

Any ideas on how I can do that?