I would try to update the Video
drivers to latest; or I would install the video
driver version that was used before that was working very well...
flash player addon is for viewing online streaming videos, and also your problem is viewing WMV files localy so, I think that is related to your video card/
drivers or your media player...
also, as you mentioned you played it in mediaplayer; what mediaplayer? is it Windows Media Player 12? or mediaplayer classic?
I would also recommend to use
VLC or if you prefer to use Windows Media Player 12, try
Shark007 codec pack
Try Internet Explorer 9 browser to see if its also corrupted.