If you've installed Windows Vista you can head off to this page to provide feedback about your experiences with the product:

    http://www.microsoft.com/windowsvista/sentiments/default.mspx

Here are Microsoft's own words about that page:

It’s quick, anonymous and will help us learn about your experience. You’ll also find this site linked inside the “Feedback” icon on the desktop after you install. We encourage you to visit it over and over to tell us about your Vista experience as you use and explore Beta 2