Just bought a brand new laptop. Put in a DVD, and opened windows media player 10, when the dvd starts you can hear the film but there is no picture on windows media player, just a pixelated mess, but when I use Power DVD, which is also installed all DVDs work and look fine and play as normal.
Personally I'd recommend you download the free VLC. It plays almost anything you can throw at it, without the hassle of downloading new codecs etc. DVDs included.