What are VirtualBox Guest Additions?
They consist of device drivers and system applications that optimize the guest operating system for better performance and usability.
To install them in Ubuntu you have to mount the CD (embedded in virtualbox) and then you have to open a terminal and type the following command:
$ sudo ./VBoxLinuxAdditions.run
If all goes well, your system will have better performance and fuctionalities, for example it will be able to use all the screen of your workstation.
If you receive a message like the following:
The headers for the current running kernel were not found.
If the module compilation fails then this could be the reason.
your installation went wrong!
To solve this problem, connect to the internet an run the following commands:
$ sudo apt-get update
$ sudo apt-get upgrade
$ sudo apt-get install build-essential module-assistant linux-headers-`uname -r` dkms
now run again
$ sudo ./VBoxLinuxAdditions.run
Great!