Net Send is such cool command comes with windows XP which you can use through command prompt or run. It is a simple command which comes bundled with windows which can send messages over Winpopup protocol.
It also depends on a windows Messenger Service which needs to be running state so that the net send can transfer the messages.
Why did Microsoft remove Net Send in vista?
As the messenger can used to spam by sending text messages which appear as message text boxes on the receiving end computer. The winpopup protocol did not have any authentication or security, which allows pop up windows to obstruct users desktops on windows.
So. the only way out left is to remove net send completely from newer versions of windows like windows vista.