<script language="javascript">
$(document).ready(function(){
$('#<%=dropdown.ClientID%>').change(function(){
var selectedval=$(this).val();
if(selectedval=="1")
$('#<%=txtfirst.ClientID%>').attr("disabled", true);
else if(selectedval=="2")
$('#<%=txtother.ClientID%>').attr("disabled", false);
else
$('#<%=txtfirst.ClientID%>').attr("disabled", false);
});
});
</script>