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!"