Hello,
Make sure the "IIS_WPG" group has "Read & Execute", "List", and "Read" permissions on the PHP installation folder (e.g. C:\php) and that the permissions are inherited on the files inside the folder (php5isapi.dll).
Be sure that you have modified the system PATH variable to include the PHP installation folder (e.g. C:\php) and that you have restarted the server after installing PHP.
To test if this is a permissions issue, you could temporarily assign the website's identity user account to the Administrators group.
Brock Hensley
Technical Support Representative
http://www.ServerIntellect.comManaged Servers, 24x7 U.S. Support, Web Hosting Solutions.