If you are using MasterPage then there is a small problem in your javascript code

Replace

var id = $('#HiddenField1').val();
var name = $('#_txtname').val();
var date = Date();
var salary = $('#_salary').val();

With

var id = $("input[type='hidden'][id$='HiddenField1']").val();
var name = $("input[type='textbox'][id$='_txtname']").val();
var date = Date();
var salary = $("input[type='textbox'][id$='_salary']'").val();

ELSE IF NOT USING MASTER PAGE, then place a breakpoint in your WebService and see if you are gettin the corrext values as your arguments