Sound But No Video

[IgSkillS1]

I downloaded American Pie 2 from Morpheus and its about 260mb in size(AVI), When I tried to play it using Windows Media Player, It said Invalid File Format. I tried to run it using Mplayer32.exe and it played the audio and it said, Video not available, cannot find 'vids:mp43' decompressor. Is there something I can download that would let me be able to watch the video, because downloading a movie like that with dial-up takes about 15 hours and what a waist of time if I can't even see the video...