So your home page (that's how it's called in IE, not default page) is your schools website, so that appears if you open IE. Then you copy paste http://www.youtube.com/watch?v=9ZTQy04EyL4 into the address bar to view our Molly Wood (don't you love her) discussing the iPhone 4s, and then what happens?
If you don't see the Molly video, but something else, it's either the video driver or a flash setting (such as hardware accelleration) that is suspected. Does this happen on all XP machines with IE7 or only on some? What happens if you try Firefox or Chrome or IE8?
I have XP Pro. and the explorer we use is Internet Explorer 7, the default page that we have is our school website and when I open a different page that has videos on the page, we can only see images of our webpage and not the video that we want to play. Can anyone please help me to solve this issue?