If you are getting the
VirtualBox can’t operate in VMX root mode error dialog shown below …
… it is because you have the kvm and kvm_intel kernel modules enabled. You will need to unload them using the following commands:
sudo modprobe -r kvm_intel
sudo modprobe -r kvm
You should now be able to start your Virtual Machine.
If you want a permanent solution, you need to disable full virtualisation for qemu/kvm using the Boot-up manager. You can do so by following these steps.
1. Install the Boot-up Manager:
You can do this via the Software Center, where you can find it in the System Tools section:
Alternatively, you can install it by typing the following in a console:
sudo apt-get install bum
2. Start the Boot-up manager from the System > Administration menu.
3. Untick Full virtualization on i386 and amd64 hardware and click Apply.
Note: VirtualBox will still run under full virtualisation provided your system supports it.