When you get to the web page and click on the download link, you see the following:
Before obtaining the requested download, please take a moment to validate your genuine Microsoft Windows installation. Validation assures that you are running an authentic and fully-licensed copy of Windows. When you select Yes below you will be guided through the short validation process. Validating now will enable faster access to genuine Windows downloads upon future visits to the Download Center. Please see the Why Validate? page to learn more about the Windows Genuine Advantage program and why validation is recommended.
This is the first time I’ve encountered this in downloading any software from Microsoft’s website. I have no worries whatsoever, by the way, as I do have a genuine and legal copy of Windows (when I ran the validation process, though, it was still with whew! when I saw the positive result).
Is this a new procedure? Part of anti-piracy efforts? I can imagine the conspiracy theorists and Microsoft-bashers will have a field day.
More info: Microsoft | About Genuine Microsoft Software