Hi,
You could try downloading a couple of video and audio codecs. My preferred ones are XVid (www.koepi.org/XviD-1.1.0-30122005.exe) and AC3 Filter (http://ac3filter.net/). Install, reboot and then try playing the vid again.
Hope that helps.
PS - If it still doesn't work download GSpot (http://www.headbands.com/gspot/gspot252dl.html), install and run the program. File>Open and then choose your video that won't play and GSpot should try to render it. At the bottom it should say if it rendered ok or not, if it doesn't click on the 'render failed' and it should provide some insight.
Chris, September 2006