1) install quick messenger in your system and add all the groups or workstations in that. so whenever you want to send the message just type the message and choose the systems or users or groups in that.. but you have to start the messenger service in all systems. through the services.msc from one system you can connect to all systems messenger services.
2) you can use the
net send systemname message
( for this also you have to start the messenger service)