Microsoft released a new support tool to monitor port usage on a computer. To quote the Web page, "Port Reporter logs TCP and UDP port activity on a local Windows system. Port Reporter is a small application that runs as a service on Windows 2000, Windows XP, and Windows Server 2003. On Windows XP and Windows Server 2003 this service is able to log which ports are used, which process is using the port, if the process is a service, which modules the process has loaded and which user account is running the process."
You can pick it up here.