As you are collecting moisture in mild weather, your windows are probably single pane, so their surface temperature is low enough to collect moisture from the air in the house. If the window surface was warmer, as with good insulated double-pane or stormed windows, the weather would have to be colder to make moisture collect. There are choices, get better insulated windows or let the moisture out of the house. Letting moisture out of the house is the more expensive and drier alternative.