Drupal on IIS 7: URL Rewrite vs. ISAPI Rewrite modules
This should help you out: Clean URLs with IIS


  The best method is Microsoft's URL Rewrite Module for IIS7, available
  via the Web Platform Installer on Windows Server 2008 and Vista.
  Download and documentation are also available on Microsoft's IIS.Net
  site. You can also use third party rewrite modules (see list above).