Hi Claire I'm not sure I understaind your situation. You say you have metal (aluminium?) storm windows? Are these triple hung? If so, the paint should not stick well to the aluminum, and you should be able to take a putty knife or screwdriver and run around where the metal sash meets the metal frame and break the paint. It should be very easy to do. My concern is that its not paint causing them to stick...rather....the house has setted a bit and caused the metal sashes to stick in the track, making them impossible to slide.
You say your original windows are not double hung. How do they open to make use of the storm windows?
The stripper won't work, except to make a big mess :-(