ช่วยดูให้หน่วยครับเรื่่อง ajax php กับ window.location ='index.php'; เมือส่งค่า Y กลับไม่ให้ลิ้งไปหน้าที่ต้องการ

<script language="JavaScript">
	   var HttPRequest = false;

	   function doCallAjax() {
		  HttPRequest = false;
		  if (window.XMLHttpRequest) { // Mozilla, Safari,...
			 HttPRequest = new XMLHttpRequest();
			 if (HttPRequest.overrideMimeType) {
				HttPRequest.overrideMimeType('text/html');
			 }
		  } else if (window.ActiveXObject) { // IE
			 try {
				HttPRequest = new ActiveXObject("Msxml2.XMLHTTP");
			 } catch (e) {
				try {
				   HttPRequest = new ActiveXObject("Microsoft.XMLHTTP");
				} catch (e) {}
			 }
		  } 
		  
		  if (!HttPRequest) {
			 alert('Cannot create XMLHTTP instance');
			 return false;
		  }
		 if(document.form1.username.value==""){
				alert("กรุณากรอก USERNAME ด้วย");
				document.form1.username.focus();
				return false;
			}else if(document.form1.password.value==""){
				alert("กรุณากรอก PASSWORD ด้วย");
				document.form1.password.focus();
				return false;
			}	
		  var url = 'check_seestion.php';
		  var pmeters = "tusername=" + encodeURI( document.getElementById("username").value) +
						"&tpassword=" + encodeURI( document.getElementById("password").value );
					

			HttPRequest.open('POST',url,true);

			HttPRequest.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
			HttPRequest.setRequestHeader("Content-length", pmeters.length);
			HttPRequest.setRequestHeader("Connection", "close");
			HttPRequest.send(pmeters);
			
			
			HttPRequest.onreadystatechange = function()
			{

				if(HttPRequest.readyState == 3)  // Loading Request
				{
					document.getElementById("mySpan").innerHTML = "Now is Loading...";
				}

				if(HttPRequest.readyState == 4) // Return Request
				{
					if(HttPRequest.responseText =='Y')*************
					{
						window.location ='index.php';
					}
					else
					{
						document.getElementById("mySpan").innerHTML = HttPRequest.responseText;	
						
						
					}
				}
				
			}

	   }
	</script>