I had the same problem on my recently freshly rebuilt vista machine running RC4. The important parts in that sentence were "recently freshly rebuilt". My install was so fresh I actually had to try and play the dvd in windows media player 11 first so that it would prompt me to set the correct region (my dvd region was set 2 and I am in region 4). After fixing the dvd region in windows the dvd's played again in Media Portal.