A partial installation of WMP 11 is, as you suggest, almost certainly the cause of the problem. As far as WMP is concerned, I can't suggest anything other than another try at installing WMP 11.
However, most of the 'regulars', here in AB's technology section, wouldn't touch WMP with the proverbial bargepole. The most popular media player, for video files, is undoubtedly VLC. It's free, here:
http://www.videolan.org/vlc/
An alternative, which many people rate at least as highly as VLC (if not more so), is GOM:
http://www.gomplayer.com/
Chris