No input file specified.

I have a windows 2000 server box, and I have installed PHP 5.2.3 and
MySQL 5 on it. I am running PHP in ISAPI mode, and I can get PHP to run when the PHP file is directly in c:\inetpub\wwwroot, e.g PHPinfo runs fine, when it's inside a file at the root level. However when I try to run it in a Virtual directory, i.e., anywhere else except the root of the werbserver, it keeps giving me the "No Input File Specified" error, withour any elaboration. I have commented out the "doc_root" in PHP.ini and have also turned on error reporting, also in PHP.ini, in addition to embedding the

error_reporting(E_ALL);
ini_set('display_errors', True);

code in the page suggested in the forum. Can you suggest anything else?