what's wrong with this om_prakash? I tried to insert your code but didn't work.
Code:
<html>
<head>
<script type="text/javascript">
function Validate()
{
var x=document.getElementById("mySelect")
if x.checked
x.disabled=false
else
x.disabled=true
}
</script>
</head>
<body Onload="Javascript:Disable();">
<form>
<select id="mySelect">
<option>Apple</option>
<option>Banana</option>
<option>Orange</option>
</select>
<input type="checkbox" onclick="Validate()" name="checkbox" value="Enable list">
</form>
</body>
</html>