Can you make sure that Smarty.class.php exists in your libs/smarty directory? It could be a case issue on your server, so make sure that it's the proper case as well. What's odd about this is the fact that if it was a path issue then it should be erroring out on the line right before the one that is actually causing the error, as shown below. So this leads me to believe it's a problem with just the Smarty.class.php file.
require_once("libs/phpmailer/class.phpmailer.php");
require('libs/smarty/Smarty.class.php');
