Most computers ship with a DVD decoder installed, which is often why Windows Media Player will have no problems playing a DVD. However, it's not provided as a part of the OS.

This software package claims to be free, and should include the software you need to play back DVD files: http://www.avsmedia.com/DVDPlayer/index.aspx
I've never tried it, so install at your own risk. Read a few reviews online before trying it out.

I've always used CyberLink's PowerDVD products (http://www.cyberlink.com/) which work well and have never done me wrong. But in all fairness, it's always come free with DVD drives that I've bought.

Mind Over Matt'er - Technology musings, opinion, and more straight from TechLore's head geek.