If you want send a message to other user in the same local network but you don’t have a 3rd party software to making LAN chat happened. Don’t worry, Windows has a emergency tool for you…
You can use net command (This tool has included at Windows 9x, Windows ME, Windows XP.) by DOS Console, here it are the steps:
1. Click Start - Run menu, type cmd in its textbox and then press Enter

or by Start Menu - Programs - Accessories - Command Prompt.

2. Type net send destination_computer_name message, for example : net send server "Just Test" (you can add double quote or not).

4. At your destination computer the dialog box that contain your message will show up.

note : If you fail try this trick, you must activate the Messenger Windows Service first. Read my article about how to activate the Windows Service.
(article no:0025)