Why can't I watch movies on my computer?

I downloaded Black Hawk Down via Torrent. The movie is longer than usual, other movies are usually of 1.5 hours. The problem is that I can't watch it on my computer as the video is stuck frequently while the sound is perfect. Other movies work fine, but the longer one's don't. The audio and video are in sync at the beginning, but the video is stuck as the movie progresses. I have 1 GB RAM and 128 MB VGA Card and processor with 2 cores. Any help will be appreciated. Thanks.