U can use MSG.exe to send messages to and from windows 7 and Vista PCs. But not sure on XP. Also think they all have to be in the same domain or workgroup if not specifying any.
MSG {username | sessionname | sessionid | @filename | *}
 [/SERVER:servername] [/TIME:seconds] [/V] [/W] [message] 
 username Identifies the specified username.
 sessionname The name of the session.
 sessionid The ID of the session.
 @filename Identifies a file containing a list of usernames,sessionnames, and sessionids to send the message to.
 * Send message to all sessions on specified server.
 /SERVER:servername server to contact (default is current).
 /TIME:seconds Time delay to wait for receiver to acknowledge msg.
 /V Display information about actions being performed.
 /W Wait for response from user, useful with /V.
 message Message to send. If none specified, prompts for it or reads from stdin.
eg; In Command Line
msg UserNameHere "Turn it off and Go to Sleep!"