function tata() {
  if (document.layers){
  width=getWinWidth();
  height=getWinHeight();
  document.layers.layback.left=width / 2 - 290;
  document.layers.layback.top=height / 2 - 100;
  document.layers.layback.visibility="show";
  }
  else if (document.all) {
     width=getWinWidth();
     height=getWinHeight();
     document.all.layback.style.left=width / 2 - 290;
     document.all.layback.style.top=height / 2 - 100;
     document.all.layback.style.visibility="visible";
  }
}

function getWinWidth() {

  if (document.layers)
    return(window.innerWidth);
  else if (document.all)
    return(document.body.clientWidth);
  else
    return(null);
}

function getWinHeight() {

  if (document.layers)
    return(window.innerHeight);
  else if (document.all)
    return(document.body.clientHeight);
  else
    return(null);
}