Do an inventory of the windows that you need to replace. Identify all old windows that need to be replaced. Don't replace some windows but not the others. It would not save you money in the long run, as energy loss would still continue.
Take measurement of the sizes of the windows. They don't have to be 100% accurate as the window contractors will likely re-measure everything. Your numbers are needed for estimates only.
Do some basic research to see what styles and materials are out there. Go to your local hardware store or home improvement store to browse around.
Estimate how much you want to spend per window. For example, for double hung windows, you can expect to spend about $300 per window if you hire a handyman or do it yourself. If you can afford to spend about $500 per window, regular contractors and small/medium sized window companies would be your best bet. However, if you have a generous budget and can spend about $1000 or more per window, you may call up the brand name manufacturers for a free in-house meeting.
Calculate how much you spend per year on gas and electricity. Let's say you spend about $250 monthly on average on your energy bills. Your annual spending would be about $3000. Studies show that about 20-25% of energy is lost through radiation and about 15% of energy is lost through loose-fit windows. Let's assume totally 30% of energy is lost because of energy inefficient windows. The annual savings would be about $900. If you plan on staying in the house for five years, then the savings will be $4500. If you plan on staying there for ten years, then the total savings would be $9000.
Calculate the tax savings. Generally you get 30% credit for qualified spending's. If you are replacing at least 10-15 windows that meet the energy saving guidelines, chances are good that you would be able to claim the maximum tax credit of $1500.
Add up the savings from Step 5 and Step 6. In our example, we will be saving about $6000 over five years of time. With the potential savings in hand, you can then decide which contractor to hire and what replacement windows to use.