$civicrm_root = 'C:/Program Files/xampp/htdocs/drupal/modules/civicrm/'; define( 'CIVICRM_TEMPLATE_COMPILEDIR', 'C:/Program Files/xampp/htdocs/drupal/files/civicrm/template_c/' );

Check the include_path in the error message. It should "include" the path you've entered for your $civicrm_root.

If it does NOT, then your civicrm.settings.php file is not being read. You can verify this by adding a "junk" line near the top of that file:

foo and foo

This is not valid PHP so you should get a PHP syntax error immediately when you try to access CiviCRM.

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