sorry, a short version
Is PHP on IIS stable enough for production use
according to zend, and logic, no.
What is the difference between CGI and ISAPI mode
CGI -> the PHP CGI binary gets loaded and initialized every time a PHP script gets executed. this means your server will be put on heavy load. when running as CGI PHP also inherits all the mad unsecurity from BlueScreen OS. ISAPI means that it gets 'loaded into' the server as a module and only needs to be initialized once. one problem with this is that Internet InSecurity is a mutithreaded server, and PHP is not threadsafe since it was designed for *nix envoironments, where a new process gets forked instead of creating a new thread. your server is going to crash even more often...
Should I use an external applications such as Zend WinEnabler or FastCGI?
sorry...don't have an answer for that one...