I'm running a little utility on my Windows Server 2003 called PortPeeker. If I configure PortPeeker to listen a high-number port (like 5678), it runs w/o error. If, however, I configure it to run at ports 21, 80, or 3389, it produces an Access Denied error. (See image.) I suspect this error occurs because there are services already listening at these ports.
Does this make sense?
If so, how do I inventory the listener services on a Windows OS?