Ok ok, sometimes crazy things happen
Host machine is Windows XP, it hosts a PClinuxOS guest with VirtualBox.
Inside the PClinuxOS guest I install VirtualBox.
Inside PClinuxOS I create another host virtual machine of MicroXP, so PClinuxOS is the guest of Windows XP, but the host of MicroXP.
Everything runs fine, but slow.
Consider installing MicroXP on modern machines should take around 10 minutes or less, well in the Virtual machine its taking many hours.
A tip, never try this, unless you have much time ;-)