Almost all guns are one hit kills to the head, sometimes they will be two hit kills if you have a machine gun or a particularly high health enemy, to get hi accuracy, crouch or press shift, then go into single shot mode.
headshot with the upgraded weapon that is second down will be an insta kill, be warned, it is not very accurate and you must crouch and use single fire to be sure of a hit
Well there's your problem. Instead of letting your laptop thaw outside, just don't freeze it in the first place!