Alright...here's what I've done. I was having trouble playing a video (on the net) in WMP. So I went to their download site and found that there isn't a newer version than 9.0 for Mac. However, there is a download from WMP that is supposed to augment QuickTime and help you play WMP files. So, I downloaded and installed that. It's called Flip4Mac.
But now, when I try to watch something on line that requires WMP, all I get is audio with a blank screen. NO VIDEO. What is going on? I've trashed WMP and reinstalled it, to no avail.
What version of OS X and Quicktime are you on? The Flip4Mac plug is for use with Mac OS X v10.3.9 or later and QuickTime Versions 6.5.1 and 7.0 or later. If you couldn't play WMP flles directly with WMP9, then it's probable that particular content was encoded with a non-Mac compatible codec, still a common occurrence. Also, Microsoft briefly had an incorrect version of the F4M plugin posted, Check that you've got v 2.0.1 not v 2.0. More info at the Telestream site http://www.wm4mac.com/wmv.htm
Free players VLC and MPlayer will often play WMP files that WMP or QT won't, so they're worth a try as well.