What is a PC?
So exactly what defines a PC?
The Hard Disc, mother board, power supply, OS, Cabinet/Case?
I would think that for most its the cabinet since all other aspects are considered upgrades -- I can upgrade my disc drive, I can upgrade my motherboard, I can replace my power supply, and surely Microsoft will want me to upgrade my OS (they get money for that one).
The only thing most people would consider a different PC is when we replace the physical cabinet/case -- so it would be logical, and in my opinion a totally defensible legal argument, that only when replacing the cabinet/case are you moving the software to another PC. This I may not like but I can live with.