In my computer the windows media player cannot play DVDs. when i play DVDs the audio is coming intermittently?

in my computer when i play dvds through windows media player i receive a intermittent quality of audio and the picture is in the 4:3 mode., i think it is because of the respective codecs for the audio and video for dvd is not installed on my computer, is this is true,? and also where do i get these codecs. Any one who knows about the websites giving these codecs, in freeware, please tell me the website address.