If you are talking about the Windows Messenger service, you can run this from a command propmt (or in the run box) by typing:
net send <computer name or ip> "text to send"
In order for this to work the receiving computer needs to have the Windows Messenger service running and you need to make sure that it is not blocked in any way by the network you are on. This service is often disabled or blocked because of a potential for spam. Type "net send ?" at the command prompt for more information.