the_teflon_don that is a very good idea and many have done similar things in the past.
While condensation should be a concern, it's not the cold air outside you have to worry about. The winter air outside is by default very dry (unless it's snowing or raining), that's why people use humidifiers in the winter and dehumidifiers in the summer in their homes.
Where condensation might come into play is, if you duct enough cold air into your PC that it drops component temperatures below the room dew point. In that case, the water vapor in the room's air might condense on your PC components.
As for your watercooling idea, yes you can get sub zero temps however you would have to substitue the coolant for something else since water freezes below 0C. You would also definitely have to condensation proof your PC in this case.