function redimensionar()
{
  var winW = 630, winH = 460;
  if (parseInt(navigator.appVersion)>3) {
   if (navigator.appName=="Netscape") {
    winW = window.innerWidth;
    winH = window.innerHeight;
   }
   if (navigator.appName.indexOf("Microsoft")!=-1) {
    winW = document.body.offsetWidth;
    winH = document.body.offsetHeight;
   }
  }
  if (winW<985){
    document.getElementById('contenedora').style.left = "0px";
    document.getElementById('contenedora').style.marginLeft = "0px";
    document.getElementById('contenedora').style.top = "0px";
    document.getElementById('contenedora').style.marginTop = "0px";
  }else{
    document.getElementById('contenedora').style.left = "50%";
    document.getElementById('contenedora').style.marginLeft = "-492px";
    document.getElementById('contenedora').style.top = "50%";
    document.getElementById('contenedora').style.marginTop = "-283px";
  }
}
anyadirEvento(window,"resize",redimensionar);
anyadirEvento(window,"load",redimensionar);