function PositionAd()
{
  var w = 0;
  var h = 0;
  var div = document.getElementById("ad_holder");
  
  //IE
  if(!window.innerWidth)
  {
    //strict mode
    if(!(document.documentElement.clientWidth == 0))
    {
      w = document.documentElement.clientWidth;
      h = document.documentElement.clientHeight;
    }
    //quirks mode
    else
    {
      w = document.body.clientWidth;
      h = document.body.clientHeight;
    }
  }
  //w3c
  else
  {
    w = window.innerWidth;
    h = window.innerHeight;
  }
  
  div.style.top = (h / 2 - 300) + "px";
  div.style.left = (w / 2 + 450) + "px";
  div.style.visibility = "visible";
}