You can use fport to determine which ports are running on a Windows machine, and which services/executables are using those ports. http://www.foundstone.com/index.htm?subnav=resources/navigation.htm&subcontent=/resources/proddesc/fport.htm.
You can use this information to determine if any services can be disabled or stopped, and to see what kind of traffic your network is creating. You should also run hardware diagnostics on the switch itself to eliminate hardware as the source of the issue that you're experiencing.
This was first published in August 2004