This might do the trick : 
http://www.cezeo.com/products/netsend/
Or perhaps try the msg command:
C:\>msg
Send a message to a user.
MSG {username | sessionname | sessionid | @filename | *}
             [/SERVER: servername] [/TIME: seconds] [/V] [/W] [message]
 
  username                      Identifies the specified username.
    sessionname                 The name of the session.
    sessionid                        he 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.
 
Syntax: msg @filename.txt type your message here