I just came to know that HC started support for FAST CGI on Windows Platform, which is great. Now my question is how we can configure it in the IIS 7.5 where PHP is already running in simple Cgi mode ?

Yes this is correct Fast Cgi support included in the HC since version 9.05. All you will need to do is to select option " Fast CGI" under Web Server Configuration in HC Admin > Cluster Configuration > Servers > Edit Web Server.
 
General guideline about FAST CGI configuration on IIS 7.x is available in this link
http://www.iis.net/learn/application-frameworks/install-and-configure-php-applications-on-iis/using-fastcgi-to-host-php-applications-on-iis