If it's windows, it will most likely be SMB/Windows File and Print Sharing/CIFS protocol (unless some crap like Netware is running). If the machines have the messenger service running then you just use the 'net send' command (there are gui versions that let you choose from the machines on the domain/workgroup to send the message to, if you're lazy) as previously stated, otherwise it won't work. Basically, its built into windows by default, unless the admins have turned it off (which if they have a clue, is quite likely).