Select Box Options To Disable/enable Checkbox?

I need to be able to uncheck and disable a checkbox based on the option selected in a Select list.

This is what I tried.

==============
<script type="text/javascript">
function GetOptVal(OptVal)
var OptVal;
{
if(OptVal="XX") document.form1.chkbox1.disabled = false;
}
else
{
document.form1.chkbox1.checked = false;
document.form1.chkbox1.disabled = true;
}
</script>
<form name="form1">
<select name="Opt1"
onchange=GetOptVal(document.form1.Opt1[document.form1.Opt1.selectedIndex].value)>
<option value=" ">Select One</option>
<option value="XX">XX</option>
<option value="YY">YY</option>
<option value="ZZ">ZZ</option>
</select>
<input type="checkbox" name="chkbox1" />
</form>