This procedure was tested on VirtualBox 3.0.12 for Windows and Ubuntu 9.10 Server 32 bit as Guest.
To be able to enjoy seamless integration between the host operating system and the VirtualBox guest you have to install VirtualBox Additional Tools on the guest operating system.
To do so, follow the steps:
Ensure you have a working Internet connection.
Install requirements for the Additional Tools:
sudo apt-get install gcc linux-source-virtual linux-headers-virtual dkms
Reboot after installation.
After logging in again find the Install Additional Tools option on the Devices menu.
Then on your guest mount the cdrom:
sudo mount /media/cdrom
Now install the tools:
cd /media/cdrom ./VBoxLinuxAdditions-x86.run
This will install the additional tools for virtual box.
Reboot your system and you’re all set.