You will NEVER save enough money on energy to pay for new windows, so don't let that be a deciding factor! I say this because window replacement is one of the most common homeowner scams, and the salesman will tell you almost anything to get you to sign a contract so he gets a fat commission.
Anyway, how you do it depends a great deal on how much money you want to spend. The best way is to replace the entire window, frame and all, but it is by far the most expensive. If the existing windows are large enough, the cheapest way is to replace the windows with vinyl pocket windows, but the glass area will become about 6" smaller in all directions (2-1/ to 3" on each side) so keep that in mind.
Pocket windows are installed by just removing the existing sashes and fitting complete new windows into the existing frames where the old sashes used to fit. New windows can be installed in about 1 hour per window, including bending new trim if they have a break on site. When finished they work and look great, but if the glass area wasn't overly large to begin with, the new windows can end up looking rather narrow.
Pocket windows are usually the best way in really old houses with large windows due to the cost factors, especially with large windows as larger windows get to be quite expensive. Using this method, you can usually get new windows for about $200 - $300 per window, installed if you use vinyl replacement windows.
Seriously, why does she want new windows? Ease of cleaning, like tilt-in? So you don't need to change storms/screens twice a year? If you answer that question, it will help you to know what you are looking for.
If the existing ones are ok, and you just don't like changing storms/screens then you might consider installing high end self-storing storm windows with screens. Something like Humphrey's are very good. It would be a great deal cheaper then new windows, and you don't need to change the storms/screens twice a year.