If you add IIS7 and ASP.NET on Vista, many of the common HTTP modules are not installed by default – including the one for static content. The problem is that it doesn’t return a 404 or other error, but rather zero-byte files. If you’re seeing empty files returned for CSS, images and JScript files, this is likely the culprit. (You’ll all see the Static Files handler for * being mapped to the DefaultDocumentModule).
The answer is to go into the Windows Features dialog and make sure that the Common Http Features are checked off.
Hope this helps.