Get GSpot.. it's free and it will tell you what you need to play a certain file.. just open the file in GSpot and it will tell you. http://www.headbands.com/gspot/

You could also get some here: http://www.divx-digest.com/software/nimo_pack.html (use Mirror2 link)

Not a bad idea to have a backup player. VLC is free & has inbuilt codecs & plays all but the odd wmv file: http://www.videolan.org/vlc/ (choose nearest mirror site)