Have you tried the shrink-plastic insulation they sell at handyman stores? Basically, it's a piece of heat-shrink plastic that you cut to fit, and double-sided tape to attach it with, and then you use a hair dryer to shrink it until it's tight and transparent. We used it when we had some single-pane windows we couldn't afford to replace, and it works pretty well, even with cracked windows. You can even attach it to the outer edge of the frame, so it would take help with everything except the gaps around the frames.
(no subject)