Find rouge computer/workgroup

I am having a strange workgroup showing up when browsing my Network Neighborhood. I have a feeling that it is an employee that has brought in some unauthorized equipment and hooked it up to the network. I have double clicked on the workgroup name to see if I get a computer name but I do not. I get a message about not having permissions. What are some methods that I can use to track this machine/workgroup down? We are running a NT4 domain with TCP/IP as the protocol. The workstations are a mix of 2000 and XP.