Replacing windows & doors usually isn't best way to save energy
With all the home energy efficiency tax credits being offered up to home owners, I have observed a predictable uptick in advertising related to home weatherization goods and services. Not surprisingly, much of this advertising is targeted towards getting consumers to buy new energy efficient doors and windows. The thing is is that for most consumers replacing doors and windows WILL NOT result in the best possible return on investment (ROI) in regards to cost vs. energy savings. For most consumers, the best possible ROI on the money they spend on home weatherization is normally to add a significant amount of insulation to their attics (up to R-50 or 15" of fiberglass/cellulous) and sealing drafts in the home (e.g. simply adding weather stripping to doors/windows, caulking door/window trim, and sealing other sources of air leaks).
Window and door sellers go all out to sell replacing windows and doors with new energy efficient ones, but it must be understood that in the most extreme cases, going from a single pane of glass to a triple pane Low-E II window only takes that window from an R-1 to an R-3.5 in a best case scenario.
The best time to install energy efficient windows and doors is either when a house is first being built OR when the existing windows/doors need to be replaced because they have reached the end of their useful service life (e.g. 30+ years old).
Unless the windows are really old and can't be effectively sealed, the most cost effective means of dealing with inefficient windows is to caulk and weather strip the windows and maybe add the clear heat shrink plastic window weatherization film over windows in the late fall. These are cheap fixes and when properly done will provide just as much energy savings as replacing the windows, at a fraction of the cost.
We recently had an energy audit conducted on our home and did a lot of weatherization work on it. In our case, upgrading our windows to Low-E energy efficient windows would have had a return of investment of 1.4% and would have taken 72+ years to pay themselves back. The best use of our money was to insulate the attic and our rim joists and reducing air heat loss (sealing wall/ceiling joints, putting gaskets behind electrical wall plates, sealing window/door trim, etc.). Our energy audit projected a ROI 34.8% with a payback of 2.9 years on properly insulating our attic, and a ROI of 29.5% with a payback of 3.4 years for reducing air heat loss. Quite simply, adding proper insulation and air sealing our house will have saved us enough money in energy costs within the next 5-7 years to actually pay for replacing our windows and doors where as replacing our windows and doors will never save enough money in our life time to pay for the other weatherization work even though the costs would have been similar.
Don't be fooled by advertising, when it comes to weatherizing your home and reducing your energy costs. Do your homework and get an energy audit before undertaking any projects to make sure the money you spend will truly save you the most possible money on energy costs. If your primary reason for wanting to replace your windows and/or doors is to save energy, take care of other weatherization projects first.