Mystery of why Boot2Docker did not work on Windows 8.1 64-bit (or Intel VT-x Virtualization errors)

I had changed to ultra notebook running Intel i7 (i7-4600U). It should support Intel VT-x. I had made sure that the BIOS setting had the Intel Virtualization was enabled. I installed Boot2Docker, which also installs Git and VirtualBox for Windows. 

Everything went smoothly initially, but when I clicked on the Boot2Docker Start icon, things started to go wrong. The VirtualBox complained of an Intel VT-x error. At first, I thought there must be something wrong with the VirtualBox software. So I uninstalled the version of VirtualBox that came with Boot2Docker and tried reinstalling an older version of the VirtualBox which I had used before.

No luck, ... it still has the Intel VT-x not supported on my CPU error.

This really drove me nuts because it was a new laptop, and I could not believe that i7 does not support Intel VT-x hardware assisted virtualization. So I checked the Intel website for the the chip specs (i7-4600U) and sure enough it says it supports VT-x.
However, this was not what I was experiencing with the VirtualBox. I downloaded the Intel Processor Identification Utility from their website just to have a way to double check the VT-x support. Sure enough, the Intel utility also reported that VT-x was off (but it had a note that said I should check the BIOS -- which I had already done earlier).

Finally, I was ready to give up.

Then I chanced upon something that caught my eye.

hyper-v disables vt-x for other hypervisors

http://social.technet.microsoft.com/Forums/windows/en-US/118561b9-7155-46e3-a874-6a38b35c67fd/hyperv-disables-vtx-for-other-hypervisors?forum=w8itprogeneral

Because when hyper-V is installed on Windows 8 Pro, the hypervisor is

running all the time underneath the host OS, and only one thing can

control the VT hardware at a time for stability.  The hypervisor

blocks all other calls to the VT hardware.. 

You could make a boot option to boot without the hypervisor running

and then VBox could use it, but the Hyper-V VM's couldn't run.

(source: link above)

Turning HyperV on/off with a reboot

http://blogs.technet.com/b/gmarchetti/archive/2008/12/07/turning-hyper-v-on-and-off.aspx

Turning hyper-v off with:

bcdedit /set hypervisorlaunchtype off

and a reboot. 

To turn it back on:

bcdedit /set hypervisorlaunchtype auto start

and reboot.

What is bcdedit?

http://technet.microsoft.com/en-us/library/cc709667(v=ws.10).aspx

Boot Configuration Data (BCD) files provide a store that is used to describe boot applications and boot application settings. The objects and elements in the store effectively replace Boot.ini.

Or, you can do just what I did, I went to Control Panel > Programs and Features 

and turned off the Hyper-V feature.