Windows should be professionally cleaned both inside and out, along tracks, and include screens. Clean windows will not only make the house sparkle, they will allow more light to come into the house. The more light, the larger the rooms will look. So make sure that you draw all of the blinds and pull back all of the drapes and curtains. Make sure that you, "Let the sun shine in." If the house has dark storm window screens, consider removing them and storing them in the garage. Make sure that buyers know that they are available because window screens are extremely desireable features. Who wants to live in a place where they can't open up the windows for a little fresh air? If the windows are double-paned and have a gap, most likely, the seal broken. You will most likely be asked to replace the windows with broken seals if they are found during a home inspection. You might as well do it when you discover the problem. An inspector will be able to tell whether the leaking window seals are hampering the heating and cooling of the house.