How to analyze Network utilization

I want to analyze how heavy trafic is on my network consisting of 200 client PC’s. There are times when the network is very slow, and I want to know if there are any freewares available or if windows 2000 server has this feature that can help me put this on a graph. I also know that if users listen to the radio over the internet this can slow down the network. I run Win2K as a File & print server & Exchange on it. I also have Win2k Resource Kit, but I have not been able to learn how to use it. Please help.