I use nVidia's PureVideo for DVD playback, available from the usuall places
Dispite people telling me that k-lite will play dvds I've yet to get it to do so... Still I've got it installed for the excellent real player and quicktime alternatives.
Ive been using the Storm Codec pack and havent had a problem with is so far. Its got all the codecs u need and more, it even has a codec to allow you to play RatDVD files on WMP. Here