My recommendation is to try the free VideoLanClient, available at
http://www.videolan.org/
VLC plays virtually every media format that creeps around on almost every computer and it's faster and uses less resources than any other player. It comes with it's own codecs and doesn't mess up or interfere with other players and has a lot of smart features like streaming video over a network (hence the name). It also doesn't phone home and tells what DVD you watch like WMP does. :eek: So nothing bad happens if you experience the downside of VLC: Since it is a 0.8.5 version, it may simply crash and abort on some computers.
If you want to use WMP, have a look at this site:
http://www.microsoft.com/windows/windowsmedia/player/plugins.aspx
There you'll also find a link to the nVidia DVD decoder which takes some CPU load off your computer, if you have a nVidia graphics chip in it. But I'm not sure if any of these is free.