better codec or software for MPEG-4 video?

well, i need your help, really...

many videos of bitorrent are in mpeg-4 video format, i install 3ivx D4 4.5 and it does not work, simply, not work.
Can any one recommend some good codec or software for MPEG-4 video?
i have wmp, bs player, dvx & quick time

VLC media player will play them all.
http://www.videolan.org/vlc/download-windows.html

thanks!