rotten window frames
You are best to update the windows. The more you can put into the place, the better chance you have for a buyer to purchase the home. Just leaving the windows in place that are old, have problems such as decaying frames and sills, are going to automatically send up big red flags, that there are other problems with the home, and you will never sell the place. Do yourself a favor, and spend the money to get it up to shape, not to leave it looking like a half done flip that the person did not care what appearances look like.
If this home is in the slums, yes leaving the rotting window frames will fit right in. If this home is in a area that the homes are kept up in good shape, and the mechanicals in the homes are maintained, spend the money, because the return from the sale will pay back, for what you invested into it.