no, it is not a command line utility. winpopup uses a graphical interface. though, knowing microsoft, there may be some command line options that you can send it to use it in non-graphical mode, but ...

My roommate (Joystickit here on Ars) and I have had a lot of conversations just by utilizing the NET SEND command. Often times it's because I want to talk to him in the next room over and I'm too lazy ...