DVD playback requires DVD decoders to be installed on the system.  Usually, you get these decoders when you install DVD playback software like WinDVD or PowerDVD.  If your system does not have such software installed, WMP may not be able to play back DVD's.  You can get a free DVD decoder at this link.

I don't really want a new media player.  I just want to reinstall the original decoder that was shipped with the system.  Thanks.

I have no idea what decoder was shipped with the system.  You can try putting the Dell drivers and utilities disk in the CD drive and use explorer to look around on it for anything that looks like a DVD playback program (something like powerDVD, winDVD, etc.).