Instant Messaging

I have a server/client network at my company with NT on the servers and a combination of Win95/Win98 on 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 be windows-based and 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 behelpful. Aside from Exchange Server, what are my options?