How large are the gaps?

What kind of windows are they (wood, vinyl, etc.)

Pictures would help.

Assuming that you have traditional wood double hung windows, your best method for retaining the windows and providing some modern day equivalence to energy efficiency, is to install storm windows.

The old wood windows generally have very loose tolerances, and it is difficult to install weather-stripping that will not interfere with the functionality of the old windows. A cabinet shop can build wooden window frames as simple as they build cabinet doors. The open frames can then receive glass from a local glass shop. Because this "accessory" is a fixed window frame, applying weather-stripping to the storm panel frame is simple.

Essentially, you are replicating what was done 100 years ago, except that you are adding a rubberized material to the edges, and sealing out the air flow to the interior side of the building. If you take it a step further, the wooden frames can be designed and constructed to accept a dual-pane glass unit with a low-e coating. In a cold environment, this storm panel with low-e glass would provide the same energy efficient qualities of a modern window. These storm panels would remain installed during the months that you would not normally require ventilation (i.e. the cold months). Using this method allows you to retain your original wood windows, while also achieving modern day energy efficiencies.

The trade-off to this approach is that it requires additional "maintenance" in removing and storing the storm panel accessories at the change of seasons; This is something modern windows don't require.

A cheap way of reproducing weather stripping is single sided glazing tape(norton 740).

Make sure you clean the surfaces as best as possible...

check www.sealantengineering.com for more info on the product

Thank you all for your knowledge. Already have storm windows that are 50 yrs old. I have thought about getting new storm windows and have reviewed a couple on the net but am not sure whether or not there have been any significant changes in their construction over the years that would make them any more energy efficient. Would certainly get them if that were the case. If I can make the present old wood windows more energy efficient, that would be my preference. Thanks sealant engineers, I will look into the glazing tape.

If you have storm windows that do not have moveable parts, then it is as simple as cutting a kerf into the four sides and installing a replaceable plastic compressible weatherstrip. Once installed into the opening, the W.S. should seal to the frame and block air infiltration (the common problem for older wood windows). If you have a more modern (50 years old?) metal storm panel with sliding glass and removeable screen parts, take them to the metal recycling yard and have some decent storm panels built (like previously suggested).

Adding foam or plastic weatherstripping to old wood windows will often create friction when operating in warmer seasons, and thus cause a user headaches in operation. If you rely heavily on conditioned air, then just stuff the voids with insulation and apply caulking to all voids. Otherwise, rely on your storm panels, unless you'd like to hire a very maticulous professional to incorporate a useable weatherstripping to the operational parts of your original wood windows.