function abre_url(url, ancho, alto, leftPos, topPos)
{
	var w = 800, h = 600;

	if (document.all || document.layers)
	{
	   w = screen.availWidth;
	   h = screen.availHeight;
	}
	if (leftPos==undefined) leftPos = (w-ancho)/2;
	if (topPos==undefined) topPos = (h-alto)/2;
	return window.open(url,null,'directories=no,location=no,menubar=no,status=no,titlebar=no,toolbar=no,resizable=no,width='+ancho+',height='+alto + ',top=' + topPos + ',left=' + leftPos);
}