I was migrating x64 Ubuntu VM from one VirtualBox to another (on different computer). The procedure is as simple as it can be: you copy files from one machine to another and than add new machine. When you are asked to configure hard disk, you choose “Use an existing virtual hard disk file” and set the location to those files and that’s pretty everything you have to do. But I had a problem – when creating new VM there was no x64 version to choose. First thing that came to my mind was that probably virtualization was not enabled in BIOS. I checked but that was not it. Secondly I checked if my host machine has x64 OS installed (I was 99,99% sure, but sicher ist sicher, as Germans say :)). Lastly I found this nice blogpost about the same problem I had. It said that Hyper-V should be disabled, because if it is not, VirtualBox cannot access some virtualization stuff. You can turn off Hyper-V in “Turn Windows features on or off” in Control Panel -> Programs -> Programs and Features:

 

And that exactly was my problem. After I turned Hyper-V off the selection of x64 OSs was possible. In case of the same problem you have to:

1. Check if your host OS is x64.
2. Check if your computer supports virtualization – if it does, enable it in BIOS.
3. Check if you have Hyper-V enabled – if it is, disable it.

If those conditions are met it should work.