You need a DVD decoder application or plug-in for Windows Media Player. Your system probably came with WinDVD. If you don't have a software CD that came with the system, which allows you to manually install that one program, (or the system restore disc that re-images the hard drive to the original configuration) you're stuck. You might be able to get one from Dell but it'll probably cost you. And since it's probably an older version, and an OEM version at that, you probably can't just download a trial version and enter a registration key or anything.
If you just want to buy one, WinDVD, PowerDVD and some others are available.