The recommended method to use PHP on an IIS server under Windows is using FastCGI. This method allows you to reuse resources similar to an ISAPI module but does not have the instabilities introduced through a multi-threaded PHP installation.
Microsoft has posted instructions on how to install PHP under FastCGI on their IIS.net website. You can see those instructions here:
To install PHP with FastCGI under IIS6 please follow these instructions:
To install PHP with FASTCGI under IIS7 please follow these instructions: