Why is my edited video choppy?

I am using Vegas 9 with a cannon vixia hf s-10 camera (avchd) . During the editing process I have no problems, however when I render the files for playback in the media player they become choppy. I am not sure what I am doing wrong any ideas??? By the way my PC has 8gb ram, huge hard drive, multiple quad core processors, and a HD video card with half a gb of dedicated..... Thanks to anyone who can help, this is kind of an urgent issue!