how often shouold i start my car during winter months?

[fongo]

I agree with 87BOSS351C. I've had my 93' stang (currently with 158K miles!) in the garage during the winter for the past two years. I still have the same tires and no significant changes to the car overall have been seen.
I personally start the car every week for about 20 minutes or so. Take it from a research chemist (myself! I get paid for a living to think about these things!..but I'd rather be racing and getting paid for it, right?) the amount of water condensate, the rate of evaporation of this condensate and its effect on the engine and exhaust is not big. Think about it. You warm up the engine at say 180º F or so. Water starts evaporating at around 90º F (30ºC..yep believe it or not!) and by the time you reach engine temperature, most of your water if not all is gone!
With this weather's low temperatures and snow conditions (New Yawk got hit with almost 3ft last week!), the little amount of condensation reacting with the metal alloys of the engine and exhaust to form rust is 'nada'.
Bottom line is..don;t worry. you're doing the right thing
happy and safe driving!