There not free, but if you have a DVD player it should have came with some kind of software to be able to play DVD's if you install that you can then use WMP to watch your DVD's.
Right, there are no free DVD decoders for Windows Media Player. You need a paid DVD decoder like Power DVD, or WinDVD for WMP to play DVD's. There are however free DVD Players. VLC media player http://www.videolan.org/vlc/ Media Player Classic http://sourceforge.net/project/showfiles.php?group_id=82303&package_id=84358 Even DVD Shrink can be used as a free DVD Player.