If your house is in reasonably good condition inside, esp the woodwork, one of the reasons may be the ventilation provided by the "draughty" windows. It's only recently that people have tried to do away with ventilation by blocking up draughts (ably assisted by silly regulations about how airtight a house must be, coming your way soon). All in this ridiculous quest to save "energy". Not wasting fuel I would agree with, as the resources are finite, but while government ministers can tell us how and why we should stop ventilating our living space to save fuel, yet indulge in their high-living lifestyle that allows them to justify wasting as much as they like, health should come before blocking up ventilation.
I know this is not what you wanted to hear, but rather than stop the draughts by secondary glazing, is there another way you could cope?