Your form should looks like this:
<?php $form=$this->beginWidget('CActiveForm', array(
'id'=>'your-form',
'enableAjaxValidation'=>false,
)); ?>
<?php
$list = CHtml::listData(Categories::model()->findAll(array('order' => 'cate_name')), 'id', 'cate_name');
echo $form->dropDownList("Category", 'cate_name', $list);
?>
<?php $this->endWidget(); ?>
NOTE: replace form tag with form widget