I "think" I know what is wrong. Do you have a way to play Dvd's? IE some program that reads dvds? I dont believe a simple codec will help. Dvd's consist of .vop, .bup, etc files. Divx works for .avi, .mpg, etc. You need a program like Windvd to play the dvd. I believe Windows media player will play them, after you download the digital media content extras. As far as I can tell from personal experience, windows media alone will not play dvd's without some sort of upgrade, add on etc. Try to get yourself a stand alone dvd program or check for WMP 10 updates.
Cool, if not let me know. Like I said I "think" (not 100% sure) you need something like Windvd, etc. Im not condoning peer to peer etc, but there are ways. After you install that, yuo can configure media player to play the dvds as default. Man, its been so long, but I ampretty sure thats how I did it. 4 computers, and you tend to forget what you did to which one.