Using Virtualbox (4.3.6) on an XP64 machine, I ran into the following error while trying to install a 64-bit version of Ubuntu:
this kernel requires an x86-64 CPU, but only detects an i686 CPU, unable to boot
This happens even if the host machine is a 64-bit machine. To fix this, reboot the box and enter the BIOS. Look around for a virtualisation setting and enable it. Save and exit the BIOS configuration screen.