When following these instructions you can just extract the KEY along with the VirtualXPVHD it's the legal XPmode key,so if you have a legal version of windows running you should not have to be forced to buy another licence.
Buying a licence from ebay is a grey area at best for MS anyway, it's just as "illegal"
I'm pretty sure the extracted key won't activate running under Hyper V (or any other virtualization software) because transferring it from Virtual PC to another virtualization system changes the underlying virtualization hardware abstraction layer (i.e. the "virtual hardware"). This would invalidate the key just like installing it on a different computer would in the real world.
You might actually be able to install Windows 7, install XP Mode and get it working, then upgrade Win7 to Win10 and still have an activated XP Mode. The license is still spotty there, though, because XP Mode is specifically licensed only to be run under Windows 7 with the provided key.
When you have let's say Windows 7 or XP running in a VM, can you still run certain EXEs in compatibility mode if need be?
30 day trial mode means you'd have to pay after? Wouldn't that be the safest way to get Windows then?
When you run a virtual machine, it is like a separate computer with a separate OS with a separate file system. For the most part, anything you can do on a computer you can do in the virtual machine (subject to the limitations of the "virtualized" hardware provided in the VM). So, you can use compatibility modes as needed as that is an OS-specific feature.
Don't get me wrong - there are things that don't work well on older OSes in virtual machines, like hardware dongles, but there are technical reasons for it (related to direct accessing of hardware, which more modern OSes don't allow). High performance games also don't work well on VMs unless your hardware and virtualization system allow the host video card to be passed through to the VM (you have to have specific computer processor, BIOS, and hypervisor support to do that, though).
The 30 day trial mode is exactly what it says it is - you can test the unactivated OS for 30 days before it restricts use. You have to activate a valid key within 30 days to continue using it past 30 days.
XP Mode was only licensed to run under a Windows 7 host OS. If you install it on a Windows 7 machine, it can be used forever with no purchase necessary (i.e. the XP key is licensed to that VirtualPC VM hardware). You only have to have a legal license if you move it outside Win7. If you move it outside Win7, you could also technically use it in trial mode forever by wiping the virtual machine and creating a new virtual machine every 30 days if you wanted to. That is a lot of work, though, since if you have a valid key you can activate XP using that key and it is again permanent.
You even technically could install Win7 in a virtual machine and install the XP Mode virtual machine under the virtualized Win7 (so you literally have a XP virtual machine running nested inside a Win7 virtual machine). It would also probably be dog slow, though.
So what's with Hyper V, why don't people recommend that instead of Virtualbox?
The Hyper-v virtualization hypervisor is only supported on the Professional or Workstation versions of Windows. If you have Win10 Home, you are out of luck.
I don't use Hyper-v myself, as I always used Virtualbox and VMware Workstation Professional. One advantage of those products is that they have drivers to easily support USB passthrough for USB devices from host system to virtual machine (i.e. your hardware connected to the host machine can be passed through to the OS in the VM), while Hyper V (at least it used to in the past) only supported passthrough of USB flash storage drives. I don't know if that has changed or not as, again, I very seldom mess with it given I have other good options. I'm sure an expert here will chime in about that shortly.
EDIT:
I've read that there actually may now be activation issues with Windows XP - some folks are reporting the activation servers aren't responding anymore and that the telephone number for phone activation is disconnected. I've also read it is still possible to activate it via phone though by calling the automated phone activation toll free number for a currently supported product (like Win10). So, YMMV on getting it to work.
Edit 2:
FYI: Yes, online activation for XP appears to be dead. The phone activation number still works - I was able to use the automated system which offered to text a link to an activation app that did work for XP. So, at least for the one I tried, it did work via phone activation.