The problem with cosmetic coverings is that they will often conceal deterioration and/or rot behind them. My suggestion to you is that you install an J-bead to edge the siding, caulk it to the window trim and then scrape, sand and paint the trim using a high-quality exterior primer and finish paint. This wil allow you to keep an eye on the condition of the trim as well as make the best appearance of the old trim possible.