Perhaps it's picking up php.ini from another location? Best way I know to tell is to create a file with this in it:
<?php phpinfo(); ?>
And then display that page. It should give you a lot of information about PHP including where the ini file comes from.
And there is, of course, the possibility that it's not a PHP error at all. Double-check your Windows Event Viewer's Application event log. The error might just show up in there.