Seems HBO/Sopranos uses Silverlight. The episode appears to load normally but I'm unable to start the player. I'm able to watch other videos/series normally.
Please go to www.xfinitytv.com/computer and copy the information you see on-screen and paste it in this thread so we can further assist you. Please send the URLs of the videos that you are trying to watch.
Also, are you receiving an error meassge when the video is not starting? If so, what does it say?