Thor,
The humidity also comes from the walls and the floor. They wick
moisture in from the outside. Now days, all interior concrete floors
have to have a vapor barrier under them, and all walls are sealed. Just
sealing the windows won't fix it. There are some sealers that can be
applied to interior floors and walls (check local masonry supply
store), but I don't know how well they work. Any cracks must also be
sealed (the only guarantees that you can give about concrete are 1 it
will crack, and 2 no one's going to steal it).
robo hippy