I have tried upgrading with the MSDN/Technet .iso files, and I could upgrade and activate both from activated Windows 7 and not activated Windows 7. So it will not be a problem using a non legal key or activator. And in other threads here people can confirm that the Win8 upgrade key works on a empty disk also (clean install) so then you can "upgrade" on non activated versions of XP and Vista also, and on empty disks.
If the Win 8 is not compatible with the old pc(Win XP Home & Win XP Pro) specs/config, is there any other option to use and upgrade it in another new PC without Windows? I want to use my old xp(OEM) in a new custom-made pc and upgrade to windows 8. As my old pc is not even compatible with Vista & Win 7, it is no use to upgrade the old pc to Win 8. If MS brings an option to change from one pc to another, it'll be helpful for more people who uses genuine xp and want to upgrade to Win 8 and sell/throw the old pc. Please help!!!