How to measure exterior track style storm windows, for replacement

I've used the plastic window stuff before (literally just finished putting up some a few minutes ago) and it works quite well. Another option to look into is storm windows, they go a long way to helping keep a house warmer. You can purchase storm windows in standard sizes at the usual hardware stores, or order custom fit ones if they don't have the ones you need.

There are also weatherstripping products you can get for your windows. Wired currently has a decent overview of what your options are.

Don't get to hung up on the exact size the piece of screen needs to be. simply measure the width and height of the frame and buy a piece of screen large enough. Normally it comes in standard widths and cut to length. Once you lay and stretch the screen over your frame, install the spline on one end, stretch, install sides and opposite end. An inexpensive spline roller comes in handy here. Use some sharp scissors, utility knife or shears and trim off the excess close to the spline. Done!