Silverlight is a microsoft program - one that will play video. Some videos are formated only for silverlight. I have silverlight on my computer, but it did ask me if I wanted to upgrade. I didn't and it still played. I have to use silverlight to watch video on my university web site - so it is a legit program. I would download it from the silverlight web site, rather than this video site though. Mine was free.
http://www.silverlight.net/