Although it's best to change anti freeze every once in a while, in practise most people don't. Modern stuff will go a very long time without loosing it's ability to stay unfrozen, so I wouldn't worry about it.
Besides a frozen cooling system wouldn't be affected by the level your fans on.
But remember that the more hot air you ask the car for, the more heat you're displacing from the engine into the cab. A cool engine is a happy engine (with the exception of when you're trying to start a diesel). So don't panic unless it starts getting too hot.
I don't mean to burn you smokes but, please for god's sake don't put cardboard in front of the radiator!
The original idea was to block the cooling effect of the radiator and stop it reducing engine temperature before the engine had a chance to get warm - a secondary effect was to stop the coolant freezing if the wind chill factor exceeded the antifreezes freezing point. A principle that was completely obsolete by 1930 with the invention of the thermostat and the advances in antifreeze technology by about 1940. So unless you've got 1940's antifreeze in a 1930's car, it's pointless and more likely to make the engine run hot!
I see people doing it and I want to shake them!
