Warm, wet air against cold windows = condensation.
You may have a leak of water into the car so check carpets all round. You may have had very wet clothes, children, umbrellas, dogs in the car that left a lot of mosture around.
Solution is to dry out the car's interior as much as possible - also when have the heater on drive with at least one window partially open if at all possible.
On a dry, warmish day try and run the engine and heater for as long as you can 20-30 mins with all the doors wide open. keep doing this until car's interior dries out.
Peccavi
December 2008