As Nobbie said, click on the phpinfo() link in the left column at http://localhost/
Look for "Configuration File (php.ini) Path" in the output. That tells you which php.ini file is being used.
After changing the file, you may need to restart Apache (or XAMPP).
If that doesn't fix it, check whether the setting is being overridden in httpd.conf or in an .htaccess file.
P.S. Turning register_globals on is not really a good idea, unless you're just testing something, but you probably already know that.