Javascript Example
Screen shot
Enable or Disable Dropdown

<html>
<head>
<title>Drop Down Disable</title>
<script language=\"javascript\" type=\"text/javascript\">
function enable_disable_dropdown()
{
if(document.getElementById(\"click\").checked==true)
{
document.getElementById(\"country\").disabled=true;
}
else
{
document.getElementById(\"country\").disabled=false;
}
}
</script>
</head>
<body>
<h3>Click on Check for Enable or Disable Drop Down</h3>
<input id=\"click\" type=\"checkbox\" onClick=\"enable_disable_dropdown();\"/>Click Me
<select id=\"country\">
<option selected=\"selected\" value=\"\">-- Select Country --</option>
<option value=\"1\">India</option>
<option value=\"2\">Germany </option>
<option value=\"3\">France </option>
<option value=\"4\">Poland</option>
</select>
</body>
</html>

You can download source code here