Network Messaging

I have a server/client network at my company with NT as the servers and a combination of Win95/Win98 as the clients. There are times when it would be nice to be able to send a non-internet-based message to my users to advise of things such as when our internet goes down. The ideal program would allow me to select which users to send the message to (maybe even by groups?), an option of allowing a reply if necessary and being easy to use. Inexpensive or public domain would be helpful. Aside from Exchange Server, what are my options?