Tony's correct. There are two ways to skin this cat, either lower the humidity (which lowers the wet bulb temperature - which is the temperature at which condensation forms) or make the insides of your windows warmer. Depending on the build quality of your windows this may or may not be possible. I replaced all my old double-hung windows with new Pella brand windows and I have much less/no condensation, even though I have a whole-house humidifier.

Of course when it gets really cold I do have to lower the humidifier.

I would also check your attic to make sure that your attic is venting properly. If it's not, then the window mold is the least of your problems. Condensation on the underside of your roof plywood will cause big damage. The attic should be well-ventilated (but not windy).

If you do end up purchasing a de-humidifier, be sure that you get one that can drain into the plumbing. If you have to empty it manually you'll never keep up.

-Zeke