[Hapson]

Разобрался вроде.
Как-то так:

function SerializeForm(form){
	var fields = form.elements;
	var field, name, value, type;
	var res = '';
	for(var z = 0; z < fields.length; z++){
		field = fields[z];
		name = field.name;
		value = field.value;
		type = field.type;
		if(typeof name == "undefined" || name == ""){continue;}
		if(type == 'checkbox' || type == 'radio'){
			if(field.checked){
				res += name +"="+ value +"&"; continue;
			}else{
				continue;
			}
		}
		if(type == "select-multiple"){
			for(var so = 0; so < field.length; so++){
				if(field[so].selected){
					res += name +"="+ field[so].value +"&";
				}
			}
			continue;
		}
		res += name +"="+ value +"&";
	}
	return encodeURIComponent(res);
}

Еще допилить надо