<?PHP $form=$this->beginWidget('CActiveForm',array(
    'id'=>'user-form','enableAjaxValidation'=>true,'htmlOptions' => array(
            'enctype' => 'multipart/form-data','enableClientValidation'=>true
    ),)); ?>

if(isset($_POST['ajax']) && $_POST['ajax']==='user-form'){
    echo CActiveForm::validate($model);
    Yii::app()->end();
}