Hi Kathy Moisture problems in the winter require two things: a cool surface and air full of moisture. The later is the result of a 'tight' house (minimal drafts around windows and doors, a central furnace with a closed loop combustion air system, an electric [not gas] water heater and not using an open wood fireplace......these will result in little outside air entering your house, and the inside air staying where it is), use of a humidifier or use of a ventless gas log set and/or lots of people in your house who take hot showers and cook a lot. All of these will contribute to a high humidity level inside the house. The cool surface comes from being cooled by outside air. This was a common problem with aluminum windows, which conducted the outside coolness and perpetually stayed cool, even when very warm in the house, and would therefore always 'sweat', or condense moisture. Because your condensation forms on window casings, my guess is that when they installed the windows, they did not insulate between the window frame and the 2X4 house frame. This allows cool air to come in from the outside and cool down the casings. The fix is to carefully pry off the casings and tamp this area full of fiberglas insullation (use a shingle to tamp with) or spray in some expanding foam you can buy in cans. Then reinstall your casings.