Is the condensation in between two panes of sealed glass? If so, the seal is broken and you need to have the window replaced. It won't be insulating as well as it should without the seal.

If the condensation is on the inside surface of the windows, then it might be due to excess humidity indoors. If you have a humidifier, it shouldn't be set too high in the winter, otherwise you'll see this phenomenon. If it gets bad enough, the water may even drip down from the window and wet your drywall and paint.

Another thing that will exacerbate the situation is if you have shutters or blinds that are always kept closed. That prevents the inside air from circulating into that enclosed area, which would help keep it warmer and more dry.

You should remember to get fresh air in regularely. As mentioned before, if you have a humidifier, turn it down, and if that does not help, open the window for a few minutes to get some dry air from the outside in and reduce the humidity.

For a single-pane, double-hung window, open the top and bottom a few inches each and leave open for 10 to 15 minutes. This will clear out most of the humidity without losing too much heat.

Depending on the climate where you live, you may want to look for a clothes dryer that is not properly vented or a bathroom that is not vented—if you are seeing much condensation, that's unusual, and you want to find out where the moisture is coming from before summer arrives and it gets in the walls and fosters nastiness.

we had a problem similar to this, and we were actually seeing condensation on the top of outside walls. in addition to things already mentioned, it could be poor insulation in that area. this creates a cold spot on the drywall which allows the condensation.

also, make sure your attic is properly vented, including adequate soffit vents, ridge vents, and possibly also power vents. if the air can't move through the attic, then humidity can build up.