Source http://www.virtualbox.org/wiki/Linux_Downloads

1. Add the following line to /etc/apt/sources.list:

deb http://download.virtualbox.org/virtualbox/debian lucid contrib non-free

2. Add and register the Oracle public key:

wget -q http://download.virtualbox.org/virtualbox/debian/oracle_vbox.asc -O- | sudo apt-key add -

3. To install VirtualBox:

sudo apt-get update
sudo apt-get install virtualbox-4.0

4. Add right for current user:

1. System / Administration / Users and groups
2. choose Users
3. Additional parameters / User's rights / Check - VirtualBox

If you had preinstalled KVM on Ubuntu (virtual machine for non graphical applications) where can be such proble as

«VirtualBox can't operate in VMX root mode. Please disable the KVM kernel extension, recompile your kernel and reboot (VERR_VMX_IN_VMX_ROOT_MODE)»

To solve it, you should remove KVM from kernel (for intel processors):

rmmod kvm_intel
rmmod kvm

For amd processors:

rmmod kvm_amd
rmmod kvm