Re-did an old house with funky windows ... two stories plus a half-buried basement.
Start with a Putty Chaser. It allows you to use a drill and a bit to carve the bulk of the putty out of the window. Wear a mask.
Easy Off Oven Cleaner does soften the remainder making it easier to scrape out ... OR, get a heat gun, a stiff scraper, and a firm wire brush. Be persistent, patient, and determined. It will be worth it when you are done.