function getScrollXY() 
	{
	  var scrOfX = 0, scrOfY = 0;
	  if( typeof( window.pageYOffset ) == 'number' )
	  {
		//Netscape compliant
		scrOfY = window.pageYOffset;
		scrOfX = window.pageXOffset;
	  }
	  else if( document.body && ( document.body.scrollLeft || document.body.scrollTop ) ) 
	  {
		//DOM compliant
		scrOfY = document.body.scrollTop;
		scrOfX = document.body.scrollLeft;
	  } 
	  else if( document.documentElement && ( document.documentElement.scrollLeft || document.documentElement.scrollTop ) ) 
	  {
		//IE6 standards compliant mode
		scrOfY = document.documentElement.scrollTop;
		scrOfX = document.documentElement.scrollLeft;
	  }
	  return [ scrOfX, scrOfY ];
	}


	function overlay(layer_number)
	{
		if (layer_number == undefined)
			layer_number = "";
		
		var offset;
		var aux;
		aux = getScrollXY();
		offset = aux[1];
		
		
		Element.setOpacity($('overlay' + layer_number), 0.5);
		$("overlay_container" + layer_number).style.top = offset + 150 + "px";
		$("overlay_container" + layer_number).style.left = (screen.availWidth - 450)/2 + "px";
		$('overlay' + layer_number).style.width = screen.availWidth;
		$('overlay' + layer_number).style.height = $("cuerpo").offsetHeight + 300 + "px";
		$("overlay_container" + layer_number).style.display = ($("overlay_container" + layer_number).style.display == "block") ? "none" : "block";
		$("overlay" + layer_number).style.display = ($("overlay" + layer_number).style.display == "block") ? "none" : "block";
	}