In Administration Tools, Service manager, have you tried starting the 'Messenger' service (make it automatic if currently manual). If it doesn't start the reason should be in the Event Viewer.

If the messenger service is running on the server you whould be able to type "NET SEND yourservername somemessage" in a DOS window and see "somemessage" appear as a popup on the server.

Hope this helps. I am not familiar with windows 2003 so I guessing the services and tools are the same as on other verisons of windows.

Peter