Which services are listening on my open

I work on a small (50ish users) NT4 network and I am trying to gain some understanding of security issues. I recently got hold of a copy of SuperScan to help moniter the network. It can tell me which ports are open on which machine but how do I findout which service or program is listening on this port. And how do I close the port if it is unneccessary.
I use netstat but only to see if a machine such as the mail server has an excessive number open connections.
Peter