function popup(jan, largura, altura){
  	var desktop;
 
    ehNav = false;
    if (navigator.appName == "Netscape")
		{
		 ehNav = true;
		 posLeft = ',screenX=' + (screen.width-largura)/2;
		 posTop = ',screenY=' + (screen.height-altura - 40)/2;
		}
	else
		{
		 posLeft = ',Left=' + (screen.width-largura)/2;
		 posTop = ',Top=' + (screen.height-altura - 40)/2;
		}
		
    if ((!desktop || desktop.closed)) 
		{
		desktop = window.open(jan,'big','toolbar=no,location=no,directories=no,status=no,scrollbars=yes,menubar=no,resizable=no,width=' + largura + ',height=' + altura + posLeft + posTop);		
		}
	else
		{
		desktop.close();
		desktop = window.open(jan,'big','toolbar=no,location=no,directories=no,status=no,scrollbars=yes,menubar=no,resizable=no,width=' + largura + ',height=' + altura + posLeft + posTop);		
		}
}