Well not exactly what you may be use to in MSN or AOL instant messenger. But from AutoCAD's command line it works great and very fast. All you need to do is tweak some settings on your computer and the person's computer which you are sending them to. (You can send a message only to a name that is active on the network. If you send the message to a user name, that user must be logged on and running the Messenger service to receive the message.) The Messaging is accessing the Net Send feature from windows to send messages instantly to a users screen. For more information on messaging take a look at the Microsoft web site.
To check if your messaging service is running go to your "Control Panel>>Administrative Tools>>Services>> then under the name category there is a messenger category. Double click on this setting and under "Startup Type" select "Automatic". Then simply hit the "Start Service button on the top left (Play button). Then close down the services. And that's it. Do the same for anyone else you want to instant message.
Now you need to get a list of computer names and which user is associated with this computer name. Then simply input it, into the Lisp code attached and your ready to go.
Here is the code or download the Lisp file. Download instant_message.LSP
(defun c:IM (/ draftername messagetest name)
(setq draftername (strcase (getstring "\nWho do you want to Instant Message? Mark/John/Steve/Brandon:")))
(setq messagetest (strcase (getstring T "\nType in your message")))
;;;;;;;First Letter in name...Workstation Computer Name
(if (= draftername "M") (setq name "Markscomputername"))
(if (= draftername "J") (setq name "Johnscomputername"))
(if (= draftername "S") (setq name "Stevescomputername"))
(if (= draftername "B") (setq name "Brandonscomputername"))
(command "start" (strcat "NET SEND " name " "(getvar "loginname") " Says: " messagetest " "))