为ubuntu 21.04 server配置允许从windows进行远程桌面登录的步骤如下：

1. 编辑 /etc/apt/sources.list 文件以更换源：

   打开并编辑

   /etc/apt/sources.list

   文件，替换其中的内容为以下代码：

   deb http://mirrors.tencentyun.com/ubuntu/ jammy main multiverse restricted universe
   deb http://mirrors.tencentyun.com/ubuntu/ jammy-backports main multiverse restricted universe
   deb http://mirrors.tencentyun.com/ubuntu/ jammy-proposed main multiverse restricted universe
   deb http://mirrors.tencentyun.com/ubuntu/ jammy-security main multiverse restricted universe
   deb http://mirrors.tencentyun.com/ubuntu/ jammy-updates main multiverse restricted universe
   deb-src http://mirrors.tencentyun.com/ubuntu/ jammy main multiverse restricted universe
   deb-src http://mirrors.tencentyun.com/ubuntu/ jammy-backports main multiverse restricted universe
   deb-src http://mirrors.tencentyun.com/ubuntu/ jammy-proposed main multiverse restricted universe
   deb-src http://mirrors.tencentyun.com/ubuntu/ jammy-security main multiverse restricted universe
   deb-src http://mirrors.tencentyun.com/ubuntu/ jammy-updates main multiverse restricted universe

2. 安装桌面环境：

   执行以下命令以更新软件包列表并安装Ubuntu桌面环境：

   apt update
   apt install ubuntu-desktop

3. 重启系统：

   安装完成后，重启系统以使更改生效：

   reboot

4. 安装和配置xrdp：

   重新登录后，执行以下命令以安装并启用xrdp服务：

   apt update
   apt install xrdp
   systemctl enable --now xrdp

5. 查看效果：

   完成以上步骤后，您应该能够从Windows系统通过远程桌面协议（RDP）连接到Ubuntu Server。