Do I just need to install the codecs? Or is there a process for streaming custom encoded media?
Streaming can only be achieved using codecs designed with streaming in mind, Divx unfortunately is not one of these codecs. Windows Media Services 9 (for Windows 2003) only supports the Windows Media 9 codecs for streaming via the RTSP and RTSP-over-HTTP Protocols (Just as Flash Media Server supports streaming FLV and MP4 formats over RTSP). The only to deliver DivX encoded files is via Progressive HTTP download, which means via a normal web server, I.E. IIS.
To deliver true streaming media you will need to transcode your DivX files into a format compatible with Windows Media Services. The Windows Media Encoder (now Expression Encoder) is provided explicitly for this purpose, Adobe has a compatible Encoder for it's services as well. Both encoders will allow you to encode a video feed or play list of files live and either act as a Push or Pull encoder that feeds the Media Server. In addition you can use these tools to transcode all of your files into Streaming Compatible formats for ondemand Publishing.