Soak a cloth in paint thinner, turpentine or acetone.
Rub the cloth over the varnish until it is gone. If the varnish stain has been on the window for a long time you may need to hold the soaked cloth over the varnish for several minutes to give the solution time to loosen the varnish completely.
Soak the area with clear water if the varnish is still stuck on the window. The water will help lubricate the surface to prevent scratches. Scrape off remaining varnish with a sharp razor blade.
Scrub the window with superfine steel wool if any varnish remains on the window. This will not scratch the surface as long as the window remains wet.
Rinse the area with clear water and dry it completely with a clean cloth.