I'm not sure what's causing the problem, but I have a few tips which might help:
Uninstall every codec/codec pack you have at the minute. Use a registry cleaner to remove all traces of previously installed codecs (this is important). I recommend RegCleaner (I attached this file to another post a while ago, you can find it here: http://www.daniweb.com/techtalkforums/attachment.php?attachmentid=614 ). After you clean the registry, reboot your computer, then install the latest DivX codec, which can be obtained for free here:
I recommend that you use the bundled DivX Player which comes with the codec, and I also recommend that you download Media Player Classic, which can be found here:
http://www.free-codecs.com/download/Media_Player_Classic.htm
Sometimes different media players can play files that other media players cannot. So try your files on both.
If you still have trouble, try using the following codecs:
XviD
http://www.free-codecs.com/download/Koepi_XviD.htm
FFDShow
http://www.free-codecs.com/download/FFDShow.htm
If your video file is an XviD you can try using the XviD Player, which can be found here:
http://www.free-codecs.com/download/XVid.htm
A lot of the time its just pot-luck when you get the correct combination of codecs.
I hope this is of some use! hehehe
Let me know if anything changed ;)