instant message

Net Send

Sends messages to other users, computers, or messaging names on the network. The Messenger service must be running to receive messages.
net send {name | * | /domain[:name] | /users} message

Parameters

name
Is the username, computername, or messaging name to send the message to. If the name is a computername that contains blank characters, enclose the alias in quotation marks (" ").
*
Sends the message to all the names in your group.
/domain[:name]
Sends the message to all the names in the computer's domain. If name is specified, the message is sent to all the names in the specified domain or workgroup.
/users
Sends the message to all users connected to the server.

message
Is text to be sent as a message.

Good Luck
0bytes

Other posibility is a free GUI tool . I use it and offer you additional options like save/open messages, auto signature, create groups, etc.
You can find this at:
http://www.geocities.com/SiliconValley/Bay/1999/index.html
(remove any spaces)