var http_request = false;

function makeRequest(url, parameters, func) {
   http_request = false;
   if (window.XMLHttpRequest) { // Mozilla, Safari,...
      http_request = new XMLHttpRequest();
      if (http_request.overrideMimeType) {
         http_request.overrideMimeType('text/xml');
      }
   } else if (window.ActiveXObject) { // IE
      try {
         http_request = new ActiveXObject("Msxml2.XMLHTTP");
      } catch (e) {
         try {
            http_request = new ActiveXObject("Microsoft.XMLHTTP");
         } catch (e) {}
      }
   }
   if (!http_request) {
      alert('Cannot create XMLHTTP instance');
      return false;
   }

   http_request.onreadystatechange = func;
   http_request.open('POST', url, true);
   http_request.setRequestHeader("Content-type", "application/x-www-form-urlencoded"); 
   http_request.send(parameters);
}