You get condensation when warm moist air in the house contacts a cold surface. When the outside temperature is substantially lower than the inside temperature, the window glass gets cold enough for this to happen. Usually, a double glazed window (thermopane) prevents the inner surface of the glass from getting this cold. Or, in the case of single glazed windows, a storm window provides the same service. If you have either of these types of windows and still get condensation, you need to look at sources of moisture within the house--for example, a clothes dryer not properly vented to the outside, no exhaust fans in the bathroom to remove water vapor produced by showers, etc.