Hey Vert,
here is a page from the Microsoft KB on trouble shooting DVD playback in XP Media Player and
here's their general site on DVD playback.
I never use WMA for DVD playback. I have PowerDVDxp and never use anything else. You might find that some of the free DVD players that rely on haveing a decoder installed will work with your decoder card if you don't like your current app.
griobhta