I was also getting this , I am explaining in detail what happened in my case. I have my project in version control that is GIT, but the core files were not present version control.To apply the patch I created a separate branch and run it.But as the core files were not in version control when I merged that branch to my develop branch and deleted the newly created branch locally. The files modified using the PATCH also got deleted. (Because those files were modified file to that particular branch only.) Admin.php and Standard.php were two file missing from following location. \app\code\core\Mage\Core\Controller\Varien\Router
After replacing the file everything started working correctly.