Sorry, I'm not quite sure "what UPnP-AV Protocol" is exactly.
The DNS-323 goes into one of my SpeedTouch 780s free ports and a long ethernet cable goes from the SpeedTouch to a Vista laptop a few rooms away. My laptop has numerous media players and all play video and music great except for Media Player 11, which stutters on some video material.
Frankly for all I know, I might not have enough RAM on my laptop or a powerful enough video card (it's only a $600 laptop) to properly run WMP.
Windows Media Player does play MP3's nicely from the DNS-323, so at one point I had the great idea of working directly on the DNS-323 and moved my WMP music library to the 323. Well, this proved to be a pain, via basic ethernet the speed is too slow and normal WMP operations which were instant on my SATA drives, took as long as 5-minutes to complete going through the DNS-323.
So, I put my music library back and now just copy the music I want to share to the 323. I actually think that Microsoft discourages the use of network drives for WMP library storage.
Again -- love this product!!!! I'm playing my entire DVD collection on it, which I converted to DivX. I've had no issues running the DNS-323 with Vista and it was very easy and fast to set up.