virtualbox4.2.18 ubuntu12.04 The system is running in low-graphics mode

参考1：http://askubuntu.com/questions/225090/the-system-is-running-in-low-graphics-mode-error-in-virtualbox

参考2：http://www.lucasnotes.com/2013/05/ubuntu-virtualbox-4212-guest-additions.html

When the message that "your system is running in low-graphics mode" appears press Ctrl+Alt+F1 to enter the termina. Then login with your credentials, and then run the following commands:

     配置好网络，执行 sudo apt-get update，然后完成下面的步骤。

1. Before installing the Guest Additions, you will have to prepare your guest system for building external kernel modules:

   sudo apt-get install dkms
   

2. Insert the VBoxGuestAdditions.iso CD file into your Linux guest's virtual CD-ROM drive and mount the ISO for the guest additions (it can be found in the VirtualBox installation directory):

   sudo mount /dev/cdrom /cdrom
   

3. Change to the directory where your CD-ROM drive is mounted and execute as root:

   sudo sh ./VBoxLinuxAdditions.run

重新启动系统  sudo reboot。