Gray Screen Playback Of Video

Been using torrent sites to download weekly episodes of my favorite tv show with no problems in playback until this week. Have tried at least six different torrent sites and when attempting to playback I get a gray square in my windows media screen and nothing else, not even sound. What could have happened. Only changes I have made since last download was updating Windows Media player and changing my antivirus software to Avast from AVG. Other downloads for previous weeks still play fine.