Replacing Your Windows
Let's say you need to replace the regular windows. Here's what you want to look for. Definitely get the kind filled with argon gas between the panes.
 You also want low-e coatings (that means low emissivity for those who care) on the glass. Southerners who are worried about summer cooling costs want low-e windows with low solar heat gain coefficients (SHGCs); Chilly northerners want them with high (above 0.70) SHGCs. SHGC values usually range from around 0.4 to 0.9.
Wood, or vinyl frames insulated with fiberglass, are your best bet no matter which climate. Look out for vinyl frames that actually are aluminum with a vinyl covering. Those will conduct the outside temperature directly into your house. Aluminum frames will do that.
You also want regular windows with something called the "air leakage rate" in the range of 0.01 to 0.06 cfm/ft.
Everybody wants to get windows with long warranties against the loss of the air seal. If the window has a U-value or U-factor rating, get a window with a low rating.
If you are looking into storm windows (and there weren't any hiding up in the attic or the garage) there are both interior and exterior storm windows available. If you choose the convenient aluminum-framed combination storm/screen windows, which most people do, be sure to get a tight unit. Don't get any unit that has an air leakage rate higher than 0.3 cfm/ft. You also want to get low-e coatings (remember low-e?) and argon gas between two or more panes of glass.
If this all has you throwing up your hands and running in terror from the room, do yourself a favor and look for windows (or doors or skylights, for that matter) with an Energy Star® label. These products are designed and labeled to meet the needs of one of three broad climate regions in the U.S: northern, central, and southern. You still want to compare Energy Star® labels to find the lowest U-value and the best SHGC for your region.
Finally, a few words on installation: do not try this at home! Get an experienced contractor. Otherwise you're probably wasting all those good coatings, coefficients leakage rates and U-values.
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