When it's wet or cold outside your windows will fog or mist up, which tends to reduce visibility :) The basic problem is that there is water in the car, either physically because there's a leak (or because you are wet, from rain, showers, etc), or it's just humid and there is mosisture in the air. If there is actually water in the car, then you need to fix that problem otherwise no amount of fiddling is going to eliminate all the moisture.
Some tips to help minimize the problem:
- Set the intake to draw air from the outside, not recirculate.
- Set the temperature setting to hot.
- Turn the blower to full on.
- Make sure you have set all air to blow to the windscreen. Point the side vents to the side windows. Close the center vents.
- Turn the air-conditioning on. This will dehumidify the air going into the cabin, reducing the water content in the cabin air.
You could also try using some anti-fog spray (from auto-stores). You can also get a container of silica-gel and put that near the windscreen, which should lower the air moisture content.