I see, that's scary. What if the HDD fails? You're SOL?
You are allowed some leeway in what can be replaced. For example, a video card change or a drive change won't triger things. But both at the same time may. A motherboard swap will also require a re-activation, but if you call MS and say that your PC died and had to be repaired they won't give yo a hard time, will just give you a new code to use.