I know your question is about how to transport your desktop PC, but what do you use a 7 year old PC for that requires you to still keep it?

It obviously costs some money, but a modest Chromebook can be had for $200-$300 and would be much easier to transport. If you move 2-3 times a year, that should pay for itself in just a couple years after checked bag fees, packing materials, and general headache of packing and moving your desktop.