Unless there are specific defects with the windows, you will likely see a bigger return on your investment with new carpet. Buyers are picky in the current market, and most want a house that will not require major projects (even painting or replacing carpet). Whatever you can do to make your property stand out from the competition can work to your advantage. If you do choose to replace the carpet, make sure you choose something neutral (not too light) that will appeal to your targeted buyer. Best of luck!