In the website's property sheet under Home Directory > Application Settings, press the <Congigure...> button.
On the App Mappings tab, click on <Add...>.
Under Executable, enter:
<pre>path\to\php\php.exe %s %s</pre>
under Extension, enter the extension you would like to use (ex. <pre>.php</pre>)
Leave the other settings at default and you are all set - but don't forget to remove the ISAPI Filter entry!
Have fun!
Brett