Assuming this is Windows XP, Windows Media Player does not include a dvd decoder.
I suggest using VLC or Media Player Classic to play your dvds. Both are free to download and use.
Two commercial programs that are commonly used are PowerDVD and WinDVD. Once you have installed one of them, WMP can make use of their dvd decoder, but you would be better off just using those programs to playback dvd video.