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