These sorts of problems are really difficult to solve over a forum like this because there are so many different things on your PC that could be cause problems. And yes, you will see many similar problems reported here, but that just reinforces the fact that the problems are most likely on your PC. If it was something on the Comcast web site, it would affect hundreds of thousands of people.
One thing you can try is erase your internet cache from IE and check all the various secutiry settings and policies.
I'd suggest downloading a different web brower and see if you have the same problem. Personally, I use the Chrome browser and have not had any problems watching things from the xfinity web site. Chrome has it's own internal version of Flash that might work better. Others have had success using Firefox. You might try both; it will not hurt you to have all three browsers installed (I do myself).
Good luck!