Computer reboots when watching videos

I have XP Pro SP1. When I go to apple.com to watch movie trailers my comp. reboots. It happens as I click on the “click here to play movie” and the trailer starts playing. I have uninstalled and reinstalled Quicktime and also changed video cards (not for this reason but because I got a better card) and the problem still occurs. I now is not the video driver so what else could it be?