In the php folder which if you did a standard install of XAMPP then those two files are in the c:\Program Files\xampp\php folder but those are files that come with the PHP distribution that is packaged with XAMPP because it ships with both PHP 5.1.1 and PHP 4.x.x.
The Apache server needs to know where the php.ini file is and with a standard default install using Apache as a service and MySQL as a service and File Zilla as a service you will find the php.ini file in the apache/bin server and it will be called php.ini not php5.ini the reason for this is to differentiate between the two. So to turn register_globals on you need to edit that file it is contained in c:\Program Files\xampp\apache\bin\php.ini.
After you edit that file then you must either restart your Apache by using the XAMPP Control Panel by clciking Stop and after Apache has stopped in the control panel then start it back up then clcik Admin and do a php.info() and you should see the changes.
Or you could just restart your computer and that will restart the Apache for you.