Silvertlight from NBC is different than youtube videos because its not using Flash 10. There are a number of tools that can rip flash 10 but not so much for Silverlight from Microsoft. The way that Silverlight differs is that its not streaming you 1 complete file its actually sending you burst data based on your connection, kinda transcoding on the fly if you will, there are tools that can capture this but when your done you may have 200 files of a few seconds each, which is useless at best, chances are they won't even be the same bitrates.
Sure you can screen record if thats acceptable to you, since you seem to be looking for a free solution try
CamStudio. I haven't used it myself, but it comes up a lot when people want a free
screen recorder.