<div id='wait'></div>
<input type='text' id='field'><br>
<input type='button' value='Отправить' onClick="Send()">

<script>
function Send()
{
   var link;
   var param;
   var txt = document.getElementById('field').value;
   if(txt=="") {alert("Поле не заполнено"); return false;}
   if(window.XMLHttpRequest) {link = new XMLHttpRequest();}
   else 
   {if(window.ActiveXObject) 
      {
      var XMLHTTP = ["MSXML2.XMLHTTP.6.0","MSXML2.XMLHTTP.3.0","MSXML2.XMLHTTP","Microsoft.XMLHTTP"];
      for(i=0; i<4; i++)
         {
         try{link = new ActiveXObject(XMLHTTP[i]); break;}
         catch (e) {}
         }
      }
   }
   if(!link) {alert("Извините, но Ваш браузер не поддерживает Ajax"); return false;}
   param = "string="+encodeURIComponent(txt);
   link.open("POST","/jquery.php","true");
   link.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
   link.onreadystatechange = function() {DataRequest(link);}
   link.send(param);
   document.getElementById('wait').innerHTML = "<img src='/images/loader.gif' width='12'>Загрузка...";
}

function DataRequest()
{
   if(link.readyState == 4)
      {
         if(link.status == 200)
         {
            var wait = link.responseText;
            document.getElementById('field').value=" ";
            document.getElementById('wait').innerHTML = wait;
         }
      }
   else
      {
      document.getElementById('wait').innerHTML = "Ошибка";
      }
}
</script>

{DataRequest(link);}

DataRequest()