In Javascript you would go something like
PHP Code:
function test(){
if(document.forms["formname"]["rdostaightshift"][1].checked==true){
document.forms["formname"]["cboStart1"].disabled=true
}
else{
document.forms["formname"]["cboStart1"].disabled=false
}
}
<form name="formname">
<input type="radio" name="rdostaightshift" onclick="test()">
<input type="radio" name="rdostaightshift" onclick="test()">
</form>