The best way is to get it to raw wood. Even after using a paint stripper there will be the need for steel wool and sanding. The dust will contain lead. Also keep in mind before applying the paint remover you have to first scrape off any loose or peeling paint. Once you have it stripped, sanded, and cleaned of all residual paint remover I would wipe it down with mineral spirits. Then prime it with a good exterior oil base primer with a little bit of paint thinner to get it to penetrate a little deeper into the wood. I'd get a good coat of that on and two coats of a good exterior latex paint, making sure to move the window often as it dries. Allow ample drying time between coats, overnight is best between coats even with latex products for a longer lasting job.
That type of work is labor intensive and can be a little pricey. If you're concerned with handling lead and hazardous materials hire someone.