Not sure what version of X-Cart you are using, 3.5.10 you can find it in include/register.php:
Code:
$fillerror = (empty($uname) || !empty($error) || empty($passwd1) || empty($passwd2) || ($passwd1 != $passwd2) || empty($firstname) || empty($lastname) || empty($b_address) || empty($b_city) || (!empty($states) && empty($b_state)) || empty($b_country) || empty($b_zipcode) || empty($phone) || empty($email));