Please help me with installing AJAX on IIS configured for PHP.

I am using xmlHTTP_send_post() in my php web-app to pre-fill data into forms and its not working for every form in the web-application. It has more than 100 forms and none of them work, the input fields are blank of say undefined!

Iam guessing that I need to install AJAX support or some extension on IIS server ... please guide