A decoder is software that "decodes" or reads the DVD and converts it to a format your computer can understand. Microsoft offers decoders for WIndows Media Player as Plug-ins available at this address:
[They are not free. The top ones are for Windows Vista and the bottom for XP]
http://www.microsoft.com/windows/windowsmedia/player/plugins.aspx#DVDD ecoder
You can search the web for free decoders, as I have found one here:
http://www.freedownloadscenter.com/Multimedia_and_Graphics/MIDI_Player s_and_Editors/Codec_Pack___Advanced.html
[It's freeware meaning LEGALLY free software and contains a number of audio/visual codecs as well as a DVD decoder]

