function openWindowCentered(){
window.open('viewcart.php', 'newwin', 'width=500, height=300, left='+(screen.width-500)/2+', top='+(screen.height-300)/2+', resizable=no, scrollbars=no, toolbars=no, location=no, directories=no, status=no, menubar=no, copyhistory=no');
}