var http = false;

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

function replace(element, file) {
  showProgress(element);
  http.open("GET", file, true);
  http.onreadystatechange=function() {
    if(http.readyState == 4) {
          //code to process Ajax request
      hideProgress(element);
      document.getElementById(element).innerHTML = http.responseText;
    }
  }
  http.send(null);

  function showProgress(name)
{
    prg = "<div style='text-align: center'><IMG SRC='images/prog.gif' alt='Loading' /></div>";
    progressViewer = document.getElementById(name);
    progressViewer.innerHTML = prg;
}

function hideProgress(name)
{
    progressViewer = document.getElementById(name);
    progressViewer.innerHTML = "";
}
}