I would say it depends on how old your windows are. If they are from pre-1990, I would suggest purchasing new windows. Window construction and window glass have come a long way in the past 20 years. Depending on which window you choose, you could save a significant amount of money every year on your heating costs while improving the appearance of your home. When purchasing be sure to ask questions about:
Window material: Wood, vinyl, and composites all have different properties.
Glass: Many companies have different coatings that are applied to the glass to maximize energy savings.
Screen: How visible is the screen when you look out of it?
Installation: Who is actually installing the window and what is their track record?
Renewal by Andersen windows:
With tax credits available and technology of new products I would recommend new windows. If your budget is really low storm windows would be less expensive but add extra cleaning and parts to operate.
Do your research on energy, types of materials, installation. There are so many options, try to find some dealers that carry woodclad, fiberglass, and vinyl products. They should be able to explain the difference between products and options available. As a window dealer myself I find it very important to help customers narrow down options to make the right product choice for their homes.
Remember even the best product is only as good as its installation.