Heater/Defroster

Hi! When I turn on the heat and/or defroster plumes of brown smoke come out of the vents. I've tried letting it run for 20minutes or so, and it doesn't stop. My windshield "fogs" up with a tough gray film when it is colder than 45 degrees outside and I can't defog the windows because of the smoke. Also, running the heat/defroster seems to make the fogging worse. I've tried to turn on the AC but that makes it worse as well. The only way for me to see while driving is to drive with the windows completely down (which can be miserable in thirty degree temps). Thanks for your help!