$('#user_input, #pass_input, #v_pass_input, #email').bind('keyup', function() {
if(allFilled()) $('#register').removeAttr('disabled');
});
function allFilled() {
var filled = true;
$('body input').each(function() {
if($(this).val() == '') filled = false;
});
return filled;
}
JSFiddle with your code, works :)