[ubuntu] ubuntu 16.04 does not detect pentium G620 as 64bit (VM)

I think you have one BIOS/UEFI setting to deal with and one misconception about VirtualBox.

First, you will need to be sure that you have virtualization enabled via your BIOS/UEFI menu. For Intel hardware, that is called VT-x. You'll have to see if your motherboard allows you to enable it.

Second is that in general you don't install a guest OS somewhere and then open it up with VirtualBox. The guest OS should be installed in VirtualBox.

So, first let us know if you can set VT-x in your BIOS/UEFI menu and then we'll see about using VirtualBox.