You need to get a device that will tell you what your inside humidity levels are. They only cost about $30 - $50 and will tell you if high inside humidity is the problem. This is most often the problem. Then you can deal with the humidity levels. Do you have a humidifier attached to the furnace? Also could be the poor ventilation of the house in general.
My list of soluions. 1. Check humidity level. Should be around 35%. Use the bathroom ventilation fan when takeing showers etc. 2.Remove any air deflectors on the heating vents and let the air rise up over the windows the way it was designed. 3. Open the drapes to let the air flow over the windows.
The attic ventilation should not affect the internal house humidity very much. Good idea for the attic to be properly ventilated if you notice moisture up there. May need to consider an air-to-air heat recovery ventilator system to bring fresh air into the home while taking out the stale/moist air.
[This message has been edited by LDoyle (edited December 04, 2002).]