Thanks for your reply, Bruce. The storm windows are some kind of metal--not sure what kind since they're painted. The paint does peel off fairly easily from the frames of the storms. However, my original suspicion was that they were not merely put in and then painted, but that they were painted and then when wet, put into the frames. This was the only explanation I could think of for why they would not open, even after I scored the paint on the edges and losened the bottom edge. But it does seem unlikely that anyone would do that.
There are two storms (top and bottom) and a screen. My windows themselves are original and are in two parts. The top part is stationary and does not move at all. The bottom part slides up and down on a chain or rope pulley.
You said that the house may have settled. Would this mean that I would have to remove the whole storm window and start afresh? This explanation sounds likely to me, because my house is a row house that was once attached on both sides. They knocked down the house on the south side, however, and the house does seem to lean a bit (although, oddly enough, to the opposite side--i.e., the side on which the adjacent house still exists.) The leaning is not huge, though; the doors and windows still fit snugly for the most part.
Again thanks for the reply, hope to hear from you again!