Whenever I try to download a video file, any movie whatsoever, if I try to preview it I get this message:

"Windows Media player could not play the file. One or more codecs required to play the file could not be found."

Does anyone know how I can find out which codecs I need? Or are the file just corrupt, cause if they're corrupt, that's a lot of corrupt files. Any help is greatly appreciated. I don't want to download a 700 MB file and find that I can't watch it.