Having spent a great deal of time working and living in 80 to 100 yr old houses in the last 30 years, I agree with all Paul said. And the putty knife is probably the best tool to start with. I found a small and quite thin pry bar that I use as Larry described. It is described as a pry bar to remove trim. May have got it from Harbor Frt. Usually it can be driven (carefully) under each corner and free the window much better than using a hammer inside. Then go around both outside and inside with the putty knife and, with a little work, most will come free -- other than being nailed shut.
One other thing to consider before doing replacement windows is to find out if the house is in any kind of historic district. Districts usually have guidelines for the repair or replacement in-kind to maintain the integrity of the building.
Have patience and good luck. Old houses can be a joy to live in, but a frustration to work on!
Roger from Missouri
If you always do what you've always done, then you'll always get what you always got!