somehow something got lost when you went back to windows vista.
For sure, you don't have to pay anything to get that back
If you really want to use windows media player and dvd playback is the only thing you want to add, then you can install something like this:
Free-Codecs.com :: DScaler MPEG Filters 0.0.8 : Free MPEG2 video decoder codec. Install Dscaler MPEG Filters and play MPEG2 files
Should do the trick.
Other than that, I could recommend a codec pack such as cccp
comes with mpc home cinema and zoom player, but you can still use those codecs with windows media player if you prefer) or vista codec package
if you want to install codecs for more formats as well, or some people would recommend a player with built in codecs such as vlc, gomplayer or kmplayer.
But for your specific issue, a mpeg2 codec should suffice, but you probably open other video files as well, so thought I'd recommend more.
let me know how this goes