เรื่อง Ajax กับ Disable อีกครั้งขอคำแนะนำจากทุกท่านด้วยค่ะ

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

	   function check() {
		  HttPRequest = false;
                  test = 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;
		  }

		  var url = 'preupload_check.php';
		  var pmeters = "tUsername=" + encodeURI( document.getElementById("filename").value);

                       //document.getElementById('upload').disabled = true; //// Disable ///

              
                        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("mySpan1").innerHTML = "..";
				}

				if(HttPRequest.readyState == 4) // Return Request
				{
					if(HttPRequest.responseText == 'Y')
					{
						window.location = '...';
                                         //     document.getElementById('upload').disable = true;
					} 
					else
					{
                                    //            document.getElementById('upload').disable = true;
                                                document.getElementById("mySpan1").innerHTML = HttPRequest.responseText;
                                      }
				}
			}
                      
	   }
	</script>

<?
	$strUsername = trim($_POST["tUsername"]);

        @session_start();
        @$_SESSION['username']="$username";


	$objConnect = mysql_connect("localhost","root","1234") or die("Error Connect to Database");
	$objDB = mysql_select_db("sigdb");

	$strSQL = "SELECT servname,Filename FROM preupload 
        WHERE servname ='$username'and Filename like '%".$strUsername."%'";

        $objQuery = mysql_query($strSQL) or die ("Error Query [".$strSQL."]");
        $objResult = mysql_num_rows($objQuery);
          if(mysql_num_rows($objQuery)== 3)
	{
echo"true";
}
	else
	{
echo"false" ;
}
	mysql_close($objConnect);
?>