var subwindow;
function popupWindow(aDocument) {
    if (!subwindow || subwindow.closed)
    { 
        subwindow = window.open(aDocument,"popup",'width=800,height=700,scrollbars=no,resizable=no');
    }else{
        subwindow.document.location.href = aDocument;
        subwindow.focus();
    }
}