用JavaScript如何获取客户端的操作系统是window2000还是window xp?

    var nav=window.navigator.userAgent;
    var osVFull=nav.split(";")[2];
    var osV=osVFull.substr(osVFull.length-3,3); 
    switch(osV)
    {
        case "5.0": 
            alert("是2000系统"); 
            break;
        case "5.1":  
            alert("是xp系统"); 
            break;
        case "5.2":  
            alert("是03系统"); 
            break;
    }