Pop-up messages in Windows:

In Windows 3.x and 9x, pop-up messages were handled by a cute little program
that came with Windows called WinPopup. With this program, you could send
pop-up messages to other computers on the LAN. In addition, this program had
to be running in order for you to be able to receive pop-up messages.

With Windows 2000, that all changed. Windows 2000 uses an all-new command:
NET SEND.

The NET SEND syntax is simple:

net send computername message

For example:

net send SALES14 Hey! Let go out for lunch!

You can use the * wildcard to send to all systems on the network. For
example:

net send * Attention all users: The Matrix has you!

The NET SEND command is part of the "messenger service" (not to be confused
with the well-known instant-messaging software MSN Messenger, which it is not
related to; This messenger service is related to Microsoft Exchange).