What software are you using for dvd playback? People seem to have more problems with the built in capability within Windows Media Player in Vista or Win7 than with some of the commercial programs. (Windows XP has no built in dvd decoder and relies on third party programs).
PowerDVD, WinDVD and Arcsoft Total Media Theater are some of the commercial players.
Free players include VLC, Media Player Classic HomeCinema and KM Player. There a few others.