The only thing I can recommend for this is carefully reading over the "install.txt" file that comes with the PHP download. This goes over most cases, and gives some fairly specific installation instructions for IIS. (I'm sure you've already gone over that though - but it's the only thing I can think of).
Is your server running correctly without PHP? - ie http://localhost/
If your having problems with IIS/NT - you might want to try out the Apache Web server. The install for that one is fairly staight forward as well - and usually it's easier to get PHP running with that. You can have two servers on the same machine (Just not running at the same time) - so if your just using it as a development platform - you can switch between IIS/ASP and Apache/PHP relatively easily. Although you should be able to get PHP/IIS working.