var http = false;

if(navigator.appName == "Microsoft Internet Explorer") {
  http = new ActiveXObject("Microsoft.XMLHTTP");
} else {
  http = new XMLHttpRequest();
} 

function replace(url, id) {
  document.getElementById(id).innerHTML = '<img src="/images/loading.gif">';

  http.open("GET", url, true);
  http.onreadystatechange=function() {
    if(http.readyState == 4) {
      document.getElementById(id).innerHTML = http.responseText;
    }
  }
  http.send(null);
}