http://img59.imageshack.us/img59/4526/nissanaltima25s2009dash.jpg
Look for a button like the one I drew arrows pointing at. It looks like a car with a little arrow pointed in a circle at itself inside of the little car. Now that button means that the car will suck air in from inside the car, heat it up, then blow it out. If that button is not on (there's usually a light on it to tell you if it's on or not) then your car is sucking in cold air from outside and trying to heat it, but this is horribly inefficient and just doesn't work. The only two reasons they have this button is 1) it's good to have it off if there's something really foul smelling in the car, you just open the windows and turn on the air, and it blows the smell out of the car and 2) when you need to defog your windows, well that fog is caused by a difference in the air temperature inside and outside the car, so by blowing air from outside of the car at the inside side of the windshield, it will defog the windshield. Anyways, so just make sure you have that button on so that it will stop sucking in cold air from outside.
