When lightning strikes or the earth wire (from the wall outlet) suddenly gets charged negatively for whatever reason, then, in theory, can it cause ESD-damage to a PC?
Because the grounding wire (inside the ATX power cable) connects earth directly to the motherboard's ground-plane.
If earth is more negative, the ground-plane accepts electrons from earth. Can this charge move too fast (if the potenial is large enough) and cause ESD-damage?
PC components are protected in a Faraday cage.
A sudden charge applied to a metal-cased personal computer will quickly come to reside on the outer surface of the metal case; this is because charge repels. For very short times, a narrow wire (grounding wire inside the case) may be a flow constriction, but most PC internal construction includes a grounded metal case around the power supply (which is the terminus of the incoming ground wire), so the likely place to look for any transient damage is not the logic board, but the power supply.
A damaged power supply might overstress any and all electronic parts to which it is connected. Lightning, nearby, might generate enough radio frequency energy to put damaging currents into peripheral (USB, video, etc.) wires, which is why they are equipped with grounded shields.
