remote access data on windows XP

I manage a LAN of 5 PC’s. I want that if I wish to send any fo the PC to send a massage by means of a POP up from a pc to other pc. I have learned that there is a method, if we go in run and type a massage to any perticular pc that masage is shown on that PC by means of a POP up. How this is done. Any body please help me. Thanks in advance…..