Wet a cotton ball or rag with rubbing alcohol and press it to the paint. Hold it there for a minute or two to allow the paint to soften. Wipe off the paint. Repeat as needed. Rinse the window and wash as usual when finished.
Moisten a cotton ball with non-acetone nail polish remover. Rub it directly on the paint until the paint is gone. For larger surfaces, wet a rag in nail polish remover and scrub the surface. Rinse the window and wash as usual. Use acetone nail polish remover for enamel or oil-based paints.
Apply paint thinner to the paint by wetting a rag or painting it on with a paintbrush. Allow it to sit according to the manufacturer's instructions and then wipe off. Rinse and wash as usual.