You have not told that you are using PHP installer or trying to install manually, If you are using installer than no problem, If not than try to use installer, Its best for beginners, After installing you can find the php.ini file in system32 (C:\WINNT\system32) directory, where you can make any changes...

I have just reinstalled IIS7 with CGI & PHP 5.2.5. I have followed the tutorial "using-fastcgi-to-host-php-applications-on-iis7" . Then i did C:\php>php -info which ran, but as i scroll through the output i notice that there is no reference to Document Root. json json support => enabled...