Are you talking about newer insulated windows with two panes and an inert gas inside? If this is the case, you have a problem that is best fixed by replacement of the panes in my opinion. What has happened here is either the seal is shot, or the window is cracked somewhere. I had to replace two of mine as they weren't installed correctly before I bought the house and would up with stress cracks which undermined the seals between the layers. Since many of these windows are guaranteed for life if they fail, it only cost me the installation service call.
If you are talking about traditional double hung windows, is the condensation on the inside? Is the house overly humid?