The fastest and cheapest way to improve your windows is to pick up the plastic window kits from Home Depot, they are easy to install (you only require scissors, knife and a blow-dryer). Once installed you will notice a huge difference.
I agree with kwik6 the plastic will help out alot.You also may use a dehumidifier to reduce condesation. If you plan on renovating , look into a hrv system . (Heat Recovery Ventilater) or air to air exchanger. A hrv provides fresh air from the outside without losing heat. It greatly improves climate control and helps with humidity. It hooks up to your exsisting ductwork so installation is easy.Call your local Hvac shop. Thanks