Once started you are presented with the option to select what the new Windows 10 VM should boot from, browse to the ISO image we downloaded earlier and accept.

Once the drive is created we are ready to boot our Windows 10 machine from the ISO image we downloaded. Finally give the new drive and name and select create.Once the drive Next we allocate how much of our hard drive this machine will use, again he more the better, 25 gig will be nought to install the Windows 10 OS but you may want more space to test applications on etc. On the next screen select the default virtualbox drive format and the following screen whether to allocate all this space immediately or allow the disk to grow and use space as it is required. Next allocate memory to the Windows 10 demo VM, give it a minimum of 1 gig, the more the better so we get a nice smooth experience. Next give the Windows 10 preview virtual machine a friendly name and make sure you select Windows 8.1 as the operating system, this will configure virtual box to support the Windows 10 preview virtual machine.

Now launch virtual box and you will see the home screen, select new to start creating the Windows 10 preview virtual machine.

Virtualbox download for windows 8.1