Choppy Video

I am using Win XP. I haven’t use the computer in a long while, and now I’ve noticed that when I tried to play a video from the news website or from Youtube, the pic and sound are choppy or jerky. I tried to run virus check and spyware check, but I came up clean. I tried to reload the codecs and still the videos are choppy like the computer is waiting to buffer. How can I get video to be smooth?