Over time, rain can seep in and rot the wood around your windows. Since this isn't a major structural repair, you can fix it yourself fairly easily. Begin by removing as much of the rotten wood as possible using a scraper and a wire brush. Next, drill small holes in the soft surface of the exposed wood. Next, seal the wood with a wood harnener. This is a product that will seep into the existing wood to strengthen and waterproof it. The holes you drilled will help the hardener soak into the wood. When the hardener has dried completely, rebuild the surface of the window sill with wood putty. For deep depressions, apply two thinner coats. Let it dry. Then sand it smooth, clean the area, and apply a priner. Finally, finish the sill with exterior paint.