To prevent condensation, you need to prevent the air from reaching it's dewpoint, the temperature it can no longer hold the amount of moisture it is carrying. The can be done by 1/not allowing a cold enough surface to allow condensation and 2/lowering the dewpoint of your air.
1/Not allowing a cold surface. You did not mention if you upgraded you windows to insulated glass. This offers a warmer "Center of glass" temperature, although the edge is still cooler because of conduction through the space. Storm windows will help in either case by effectively warming the windows. Covering with an insulated curtain does the reverse by making the windows cooler.
2/ Lower the dewpoint. As Henry mentioned, this is probably the main reason for your condensation. I do however disagree with his measures to solve the problem. When houses are built tight (Which is a good thing), one needs to also include measures for air management. exchanging the air lowers the humidity and exchanges you breathing air, making the house a healthier place to be. dehumidifiers only address the moister (And not that well) and bathroom and kitchen fans only address the exhausting of air without allowing for air recovery. A study of such fans also showed them not being very effective at exhausting (In some cases as low as 30-35% of their rating).
The best solution is to have a balanced, central system that has the power to exhaust effectively, and allows for sufficient make-up air.
The best Relative humidity level to shoot for is between 40% and 60% (This based of health concerns), however, in the wintertime, you will probably need to go lower to reduce condensation.