The contents of movie DVDs are often stored in vob files. That supposedly stand for Video OBject. Assuming the contents of the vob file aren’t encrypted, you can play it like you would a regular mpeg-2 file, either with media player classic, videolan, or through windows media player using a third party mpeg-2 decoder like the elecard decoder.
If the contents are encrypted (as they are in commercial DVDs), you would need DVD software like powerdvd or windvd (you usually have to pay for those).
clarification - dvd software will also play non-encrypted vob files.