Depends on your definition of "desktop". I have all my media on an Acer Revo running XBMC on Linux, which shares the media out by uPNP/DLNA, and it just shows up in the standard media player (but, does not do transcoding by default, and N900 may not play all types by DLNA that it supports for local files). I think xbmc *can* transcode, but I really don't want to watch media I have in 720p or 1080p in 480p. It does work very well for music though.

If you don't want to run XBMC all the time, coherence (on Linux) is also able to share by DLNA (while you run a standard desktop).

For Windows, I guess Windows Media player should be able to do this too?