I don't think it's your codec install giving you problems. Get the win32 codecs from the mplayer homepage and install those www.mplayerhq.hu
Then, fire up mplayer and play your video again:
mplayer -vo xv /path/to/file/filename.avi
What are you using to play it? I mean, what command? Also, how did you install it, via RPM or did you compile it?