The antifreeze is in there to stop galvanic corrosion, caused by two metals in the loop. Copper will eat aluminium, which will degrade the radiator or whatever is alu. Antifreeze ought to be pretty unconductive and most water cooling liquids are non conductive to a certain extent (i.e. don't let them leak really, but a tiny splash shouldn't kill your pc). With the antifreeze, and the pump running, the liquid would require something insane like -40C to freeze. Unless your room is -40C I wouldn't worry (and if it is, water cooling setups freezing is the least of your worries) because water cooling can only get the temperatures down to ambient at best.