Assume you have a LAN established environment and all PC connect to a same LOCAL Domain. So if you're in WinNT platform, then you can use the Net Send command to send a message to another network PC.
Code:
Shell Net send <Parameter> <Your Message>
The Parameter can be oone of the following:
- Account
- User (Send message to a particular user)
- Group
- * (All Terminal under the same domain)
- View (View all the PC that loged into the Domain)
- LocalGroup (Check all the available User group in the Domaina)
- Start
- Stop
For more information just type Net/? at the command prompt and you should get all the above mention syntax.
For Win9x Platform, you may need to use the Winpopup.EXE (I think this the file, hope i didn't spell wrongly) 
But in order for this little gadget must lunch first before it can receive any message from others netword PC,
Hope this can help.
[Edited by Chris on 06-20-2000 at 08:07 PM]