function login()
{
    var form = document.myform;
    if(form.txtUsername.value == '')
    {
        alert('请输入用户名!');
        form.txtUsername.focus();
        return false;
    }
    else if(form.txtPassword.value == '')
    {
        alert('请输入密码!');
        form.txtPassword.focus();
        return false;
    }
    else
    {
        return true;
    }
}