For anyone else having issues with Virtualbox guests detecting ony a 32bit CPU (despite having ticked the “I/O APIC” option in Settings->System on the motherboard tab, I had the same problem, until I enabled a couple of options in the VMs settings via the command line. This was on Ubuntu 13.10 host:
sudo VBoxManage modifyvm “Test W7″ –acpi on
sudo VBoxManage modifyvm “Test W7″ –longmode on
I would guess that it was the second of these commands which made it work, but I present both “just in case”.
It turns out that unless you tell the wizard that you’ve got a 64 bit guest at the time that the VM is created (in this case, I was re-using an old one) then VB doesn’t actually enable 64bit CPUs for the guest. –longmode on (above) is what does the business in this case.