R_1 Why not submerging whole PC in liquid helium?

Because some components simply cannot run that cold. Capacitors for example may have trouble at extreme temps due to the materials used to keep the metal plates inside them apart. Plastic parts would become brittle and easy to damage, And anything using a standard gauge wire would be cold enough to be considered delicate.You also have other factors, How would you maintain a bath of Liquid helium keeping costs down, and keep connectivity to the important parts of the motherboard so you can still tweak settings. If you need to adjust, or reconnect components you also have to bring the motherboard back out of the liquid helium..Finally that much liquid helium or any liquid gas for that matter in a confined space (room) could be dangerous. As the helium could displace the oxygen... And you would perish.