[cudgy]

Original windows are likely the biggest offender. You could just replace the largest windows if budget is a concern or place plastic over the inside portion to create an “insulative” air barrier between the plastic sheet and the window. The plastic trick worked amazingly for me when renting an old farm house on a city plot. Looks ghetto ugly though.

[runnerup]

There’s clear plastic shrink wrap which truly does become nearly invisible once you run a hair dryer over it. Last one I used was “Duck” brand and worked great. People would walk up to the windows and be surprised to hit the plastic that they didn’t see.