It's most likely a codec issue. I'll suggest 2 things:
1. Get xtra codecs at either (a)
http://www.land.ee/d_nimo_codec.html or (b)
http://www.free-codecs.com/
2. Get a backup player with built-in codecs & plays everything except the odd wmv file; VLC:
http://www.videolan.org/vlc/ (choose nearest mirror site)
If you'd prefer VLC to be your main reliable player when you launch or preview video files then see:
Setting Default video app