Winterizing a house is a must. Weatherproof your windows to save on heating: If you have not already done this, then you are literally throwing money out the window. And it is not just old wooden windows that may need weatherproofing. Unless your windows are relatively new (less than 5 years old), you may be losing heat in the winter.
Most newer windows now are sealed, double or triple-paned glass with an inert gas between the panes to serve as a barrier to keep heat in during winter months. Over time those seals can degrade causing the gas to leak out. If you see moisture on the window and it is not on the outside or on the inside, it is in between the glass, and most likely you have a failed seal. Failed seals will make the window look cloudy at certain times of the day based on humidity levels and the direction of the sun. While it is not a big problem, it is no longer living up to the original insulation value assigned at the time of manufacture.
Also, be conscious of air gaps where caulk on the outside has deteriorated or pulled away. As much as 40% of energy loss can be attributed to air gaps in and around windows. Caulking the gaps can make a big difference in your heating bills and comfort level.
If you have really old windows, going to the big box stores and purchasing and installing winterizing kits that contain flexible foam sealant in a roll and clear see-through cellophane (like saran wrap) is a good idea. It is amazing how much you will notice the lack of draftiness and decreased energy bills.
Overall, saving for replacement windows is a good idea but the Fall and Winter may not be the best time to purchase the windows so shop around in an offseason.
You should also take note that this not only applies to winter months but also works the opposite direction in the Summer when poorly sealed windows will allow your cooling efforts to escape and increase your temperatures inside as well as your energy bills.
Talk to a window professional to see how they may be able to help you with winterizing a house.