Who is chatting on the network and what?

I have a Network of 80 Machines with Ip Range 192.168.0.1 to 192.168.0.254. I have a router which is the default gateway of the network , with Ip 192.168.0.4 and a leased line of 256 KBPS. All my machines use windows 2000 as the OS.My management wants to know who is chatting on the network as a lot of people spend a lot of time chatting and also want to capture the contents of chatting. I wish to have to a tool to capture this data.