Some PHP versions have some extensions built-in.
PHP 5.3.0 for example has the xml and zip extensions and others built in by default in the windows builds.
EDIT: for your error: Do you have error reporting enabled? BTW: PHP 5.3 has changed the error reporting codes, iirc
I have checked the 5.3 PHP release and there is no php_xml ext. Is there any way you could send me a copy, i'm experiencing issues with the builder:
PHP Fatal error: Class 'XMLWriter' not found in /var/www/www.clydecashforkids.com/Classes/PHPExcel/Shared/XMLWriter.php on line 66
Thanks in advance