var ventana = null;

function vent(pagina,nombre,w,h,parametros){
LeftPosition = (screen.width) ? (screen.width-w)/2 : 0;
TopPosition = (screen.height) ? (screen.height-h)/2 : 0;
ajustes = 'height='+h+',width='+w+',top='+TopPosition+',left='+LeftPosition+','+parametros+''
ventana = window.open(pagina,nombre,ajustes);
ventana.focus();
}