Try changing the user that runs the IIS processes for anonymous access: apparently the php_oci8.dll, or some other dll that is openend in cascade, is not quite happy with being opened by a user that belongs to the 'Guests' group.
I figured that out by disallowing Anonymous access and discovering that the error would be gone when accessing the page with IE: integrated authentication had IIS run its processes with the windows account i was logged in at the time, i.e. superuser. The funny thing is Apache never had any trouble running on the same machine: same php install, php.ini file and web doc root... Hope that helps Gaetano
