I installed VirtualBox 4.3.0 r89960 onto a machine I recently built (and also upgraded to Windows 8.1 Pro from 8 Pro). I hadn't tried to install VirtualBox on this particular new hardware prior to upgrading to 8.1, but it did install fine as well as the VirtualBox extensions. The hardware is an ASUS Maximus VI Hero with 16GB RAM installed and a Core i7-4770k processor. The motherboard does have virtualization enabled in the BIOS--I double checked that.
I setup an Ubuntu guest and gave it 4GB RAM and 2 CPUs. I also setup the usual stuff I have in the past with my older machine (was a Core i7-920) regarding guest system options such as VT-x, Nested Paging and PAE/NX. In spite of setting up things as I did on my old system where I could run 64-bit guests fine with more than one CPU, this one stone walls me. On attempting to boot off the 64-bit Ubuntu 13.10 desktop ISO media (I also tried 13.04 64-bit), I get this message before it gets far in the boot process at all (before any prompts for me to interact with it while it's still loading off the ISO media:
"This kernel requires an x86-64 CPU, but only detected an i686 CPU. Unable to boot - please use a kernel appropriate for your CPU."
I don't have any other virtualization technology running on this machine (that I'm aware of) - I verified Hyper-V was not enabled as a feature in the OS, and I don't have anything from VMware or other such technology running on this. Any advice what steps to take next? I ran a program I saw recommended called SecurAble and it indicated there was hardware virtualization, though it said it can't verify if it's locked or not (said it needs a 32-bit OS or something to do this). I know the i7-4770k doesn't support the VT-d technology, but I didn't think that was necessary to run a guest like this--just VT-x. I think I have everything setup as I have done in the past with my prior system.