Add one of the following lines according to your distribution to your /etc/apt/sources.list:
sudo apt-key add oracle_vbox.asc && sudo apt-get update && sudo apt-get install virtualbox-4.1 && sudo apt-get install dkms
VBoxManage internalcommands converttoraw image.vdi image.raw
qemu-img convert -f raw image.raw -O qcow2 image.qcow
sudo add-apt-repository "deb http://download.virtualbox.org/virtualbox/debian lucid non-free"wget -q http://download.virtualbox.org/virtualbox/debian/oracle_vbox.asc -O- | sudo apt-key add -
sudo apt-get update && sudo apt-get install virtualbox-3.2echo "deb http://download.virtualbox.org/virtualbox/debian maverick non-free #VirtualBox" | sudo tee -a /etc/apt/sources.list
wget -q http://download.virtualbox.org/virtualbox/debian/oracle_vbox.asc -O- | sudo apt-key add -sudo apt-get update
sudo apt-get install virtualbox-3.2