function getHTTPObject(){
  if (window.ActiveXObject) 
    return new ActiveXObject("Microsoft.XMLHTTP");
  else if (window.XMLHttpRequest) 
    return new XMLHttpRequest();
  else {
    alert("Your browser does not support AJAX.");
  return null;
  }
}

function doWork(file){
  httpObject = getHTTPObject();
  if (httpObject != null) {
    httpObject.open("GET", "ajax.php?file="+file, true);
    httpObject.send(null);
  }
}