WiMP just doesn't play fair with codecs that aren't loaded natively (you download the installer from microsoft, and what it plays without adding anything else). You can use all the ideas that were given here, as I'm sure they all work. But I found that a really small and non-resource hogging program can work really well. You may even have it already on your machine. If not, download the newest version of Winamp. I think its 2.91 right now. It has a built in video player. Just install winamp, and you may have to re-install the codec. But Winamp is what I use to watch Divx and XviD files.
http://classic.winamp.com