Easy-peasy, if you've already got PHP setup as CGI first remove that where appropriate (or modify as most of the stages are pretty much the same)
Open the IIS MMC (Start > Admin tools > IIS)
Goto the Web Service Extensions and click "Add a new web service extension"
Call it whatever you like ("PHP/ISAPI", "PHP-ISAPI", doesn't matter), and click add to add the path to the correct PHP ISAPI dll - php5isapi.dll for PHP5 obviously, etc.
Click the "allow this extension" tickbox and click OK
Expand the Web Sites tree and either go to the website or the directory you want to enable PHP on, and right click on it and go to properties
Go to the Directory tab, and click the "configuration" button
Click add and set the executable to the PHP ISAPI dll, and add .php as the extension. Limit the HTTP verbs if you wish. Make sure "script engine" and "verify.." is tick and click Ok
If you want to add phps, it's pretty much the same.
There was a problem with the ISAPI dlls a few releases ago, which meant you had to get the newer ones from snaps (snaps.php.net), but I believe that's fixed - I've not touched our work installation of PHP5ISAPI since it went in about 2 months ago though, so I can't remember exactly.
If you're still stucksies, and are around this evening I could give you a hand over MSN.
. SOLVED!