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最近我在旧机器上安装了旧版本的 Ubuntu。每当我尝试安装任何软件时，都会收到一条错误消息，说找不到它：

$ sudo apt-get install vlc
Reading package lists... Done               
Building dependency tree       
Reading state information... Done    
E: Couldn't find package vlc

将 Ubuntu 12.04 刻录到 CD 后，我尝试启动并收到消息：

This kernel requires the following features not present on the CPU:
pae

Unable to boot - please use a kernel appropriate for your CPU.

这是什么意思？