<form name="form1" action="#">
   <select name="combo1">
      <option value="A" selected="selected">A</option>
      <option value="B">B</option>
      <option value="C">C</option>
   </select>
   <input type="text" name="text1" disabled="disabled" />
</form>
<script type="text/javascript">
document.form1.combo1.onchange = function() {
	if(this.value=="C")
		this.form1.text1.disabled = false;
	else
		this.form1.text1.disabled = true;
}
</script>