If you want to remove your virtual machine installation from your Ubuntu machine due to some reason,then follow this instruction.
open your terminal by pressing ctrl+alt+t ,and type these commands.
1. $ sudo dpkg -l | grep virtualbox
which shows something like this.
ii virtualbox-4.2 4.2.6-82870~Ubuntu~quantal amd64 Oracle VM VirtualBox
2. now from above command you can determine which version of virtual-box is installed on your machine.After this type these commands in your terminal,Just replace the 4.2 with your version of VirtualBox.
$ sudo apt-get purge virtualbox-4.2 virtualbox-qt
when you press enter the terminal it asks for your permission,enter “Y” and it strated purging your Virtual-Box installation. above command is very useful as it remove all the modules of Virtual-box. As your terminal says “ureadahead will be reprofiled on next reboot” so after a simple init 6 your machine is free of Oracle VM VirtualBox