Yes, you need some codecs:
divx codec
www.divx.com/divx/
xvid codec
www.xvid.net or from builders like koepi or tom
ac3 codec or filter
windwos media codecs 8, 9
mpeg 2 codec (elecard is a good one)
To prevent wmp to play your video files you must also unasociate the player in tools/options/file asociation.