If you find yourself strangely unable to open video files, like WMV (Windows Media video) from a web page, you might need to implement the fix in KB974538.
What happened for me on my Vista laptop is that I decided to click on a video review for the new Xbox 360 Batman game (please don’t judge me). And as a result, I got an IE8 error page with the message “Internet Explorer cannot display the webpage” and then a little “What you can try: [Diagnose Connection Problems]“.
I had opened plenty of WMV files recently. What changed? Oh, I had recently installed the latest version of Windows Live so that I could enjoy the wonderful new Windows Live Movie Maker. All sorts of great and good changes happened in that update. Including, however, an itsy-bitsy-teeny-weeny break in video type registration.
So I went to KB974538, Video Files do not play back as expected in Windows Internet Explorer and other applications after installing, upgrading or uninstalling Windows Live Photo Gallery. There’s a little snippet of registry goo between those dashed lines that you need to save as fix.reg and then double click and apply.
Ba-da-bing, ba-da-boom, it’s all working again. Nice.
I hope this helps anyone else that runs into the situation.