If IIS is giving you 404s for PHP scripts, that is a separate issue and needs to be fixed. PHP does not run very well through IIS (It can do so though) and there are a number of issues which can't be easily resolved.
My best guess is that you have not enabled this extension in the IIS6 "allowed applications" panel (which is new to IIS6 and does not exist in any other version of IIS, including the crippled one which comes with XP pro).
This was essentially Microsoft finally seeing sense and realising that it's better to have functionality disabled by default and insist that the admin manually enables it.
The second problem is that the relevant PHP extension isn't loaded - but this is another issue, and until you can execute <?php phpinfo(); ?>, you don't need to worry about this.
Mark