A lot of DVD drives dont seem to come with any DVD Decoders these days.. Which is pretty suck ass.. There are a few free codecs out there but they are terrible. You'll have to shell out around $15 for a decent one.. I went with the DVD XPack, and seems to be great quality and plays all my movies fine.
http://www.intervideo.com/jsp/Produc...jsp?p=DVDXPack
Edit: That is for XP only, which I am using obviously. Just a FYI.