IIS, PHP and LANs

I am running IIS on my PC as a development platform, and all is good. I'm wanting to set up a virtual directory to a network drive on the LAN and it works fine for HTML files, but when accessing a PHP file I get The specified CGI application misbehaved by not returning a complete set of HTTP headers and no output. Surely if the HTML files work, the PHP ones will too? The server is configured will all rights and every option (r/w/exe etc.) I have seen this question bandied about now and then, but no solution has worked as yet.