You could use the Windows NET SEND command for this.

http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/net_send.mspx?mfr=true
http://en.wikipedia.org/wiki/Messenger_Service

>I have a client that would like a "Station to Station Messaging"
>system. Their are about 10 stations on the Local Area Network.
>He would like an application that would allow one station to
>send a message to another station. A popup window would open
>when a station received a message showing who the message was
>from and the level of urgency. He would like these messages
>to be logged into a database.
>
>Do you know where we can get an application like this?
>
>Thanks,
>Terry Harris