It all depends on what you mean by "stream". Like all of the formats out there now WindowsMedia support "real" streaming (rstp) and "http" streaming. In the latter all you need is a web server, and the client does all the work of caching the file and playing it while it downloads.
"real" streaming requires a Microsoft media streaming module, which is only available for IIS (which is only included in Windows Server versions). It allows for a potentially much better streaming experience (cache hinting, dropping frames when necessary, etc), but Microsoft has a nasty pricing structure for it that limits the number of clients. Oh... and if you are going to be doing a lot of DRM or live video/audio, then the media server is required.
The real question that has to be asked is do you have to have a WindowsMedia streamer? Apple's QuickTime Streaming Server is free (and based on their open sourced Darwin Streaming Server) and can stream MPEG-4, which should be able to be viewed on any number of viewers (I believe that WMPlayer is one of them). And if you add in the (free) QuickTime broadcaster you can produce live streams as well. All this without having to pay per-user fees, and you do not need to buy anything more that a computer with MacOS X on it... you don't even need the server version, although that comes with the streamer installed).
PS... always include your question in the body of the message... you can't see topics when replying.