ISAPI comes as option to select for installation, just like ASP settings, where for running Moodle, you wont need ASP, so to run PHP using FastCGI, you dont have to select ISAPI and ISAPI filters for they were used in previous gen Servers where PHP was handled by CGI.
Where you mentioned your IIS 7.5 came with ISAPI, means you haven't initiated installation yourself?
in addition, do the following
on IIS SERVER level (Not site level, as you wont find FastCGI there), go to FastCGI settings
click that, select FastCGI of appropriate PHP listed (you may have more than 1 PHP versions installed)
click Edit from Right side menu
and make sure following have values 300 (means 5 mins) or above - mentioned at point 1 and 2
