It could be simple. People are encoding in any number of formats like you see at http://www.moviecodec.com Such codecs are not endorsed by Microsoft and will not be automatically detected and installed.
You have to figure this out...
Why are there alot more corrupted downloads this year than last year? I get error messages in Windows Media player. Windows Media Player cannot play the file. The file is either corrupt or the Player does not support the format you are trying to play. It will play for a short time and then the error message pops up. It is a wmv file. I tried redowndloading and the same thing happens. I am running Windows XP Home and Media player 9.