var widScreen = window.screen.width;

<!-- ABRIR JANELA POPUP
function estudos(param) {
window.open('mensagens/' + param + '.htm','janela01','status=no,resizable=no,scrollbars=yes,menubar=no,width=640,height=400,left=' + ((widScreen - 640) / 2) + ',top=100') ;
}
// -->