Okay, I looked into your problem. Basically, WebM is a video format Youtube uses. Unfortunately, Explorer does not support it. If you click on the orange banner saying "Try out WebM Video for Microsoft Internet Explorer 9 (Beta)," it will install a plug-in to let you view the videos. However, there are some
issues associated with it.
I would recommend going with Firefox or Chrome, personally. It will solve the issues you're having without having to grab a third-party plug-in, and Firefox has solved most of the issues it had a few years ago. Otherwise, get the WebM plug-in, and you'll most likely be able to run it fine that way.