gistfile1.phtml

<!-- form  -->
<form action="" method="post">
	<select name="os" id="selec-os">
		<option value="pc">PC</option>
		<option value="mac">MAC</option>
	</select>
	<input type="text" name="os" id="os-field" />
	<input name="submit" type="submit" value="Submit" />
</form>

<!-- javascript  -->
<?php Yii::app()->clientScript->registerCoreScript("jquery"); ?>
<script type="text/javascript">
$(document).ready(function() {
	$("#selec-os").change(function(){
		if($(this).val() === "mac") {
			$("#os-field").attr("disabled", "disabled");
		} else {
			$("#os-field").removeAttr('disabled');
		}
	});
});
</script>