// --------------- parte de la api --------------------
function getObj(name)
{
  if (document.getElementById)
  {
  	this.obj = document.getElementById(name);
	this.style = document.getElementById(name).style;
  }
  else if (document.all)
  {
	this.obj = document.all[name];
	this.style = document.all[name].style;
  }
  else if (document.layers)
  {
   	this.obj = document.layers[name];
   	this.style = document.layers[name];
  }
}

// --------------- parte del centrado --------------------

function centrar(capa,ajuste){
	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;
	 }
	}
	var centro = (winW/2) - ajuste; // 100 es la mitad del ancho de la capa (200/2)
	objeto = new getObj(capa);
	objeto.style.left=centro;
}