Window glazing removal is easier using heat to soften the old glazing putty. The key to saving the glass is to heat the putty evenly, not just one area. If one area of glass gets hot it will expand more than the rest of the glass. This can cause the glass to crack or even explode.
Some tools used to soften glazing are a heat gun, glazing iron or propane torch. Some heat guns have deflector plates that direct heat off the nozzle. This allows you to heat the glazing and minimize the heat against the glass.
Besides a source of heat, you will need a good stiff putty knife and a sharp wood chisel, leather gloves and eye protection.
Do you have any experience with a propane torch? Set the flame low and heat the glazing evenly. Try to direct the flame directly against the glazing so contact with the glass is minimized. Heat the glazing several times on each rail and then rotate to the next rail. Keep moving.
Be patient and slowly heat the glazing all around the glass. As you are doing this, the paint will melt and even burn. Take precaution not to breathe in any of these fumes due to the unknown origin of the paint. Wear a mask.
Once the glazing is warm enough it should be easy to remove with a sharp chisel or stiff putty knife.
A quicker approach is to forget about saving the glass. Use the sharp wood chisel and a small hammer and chip out the glazing. Not always but sometimes the glazing comes out in big chunks. There is still the danger of getting cut from the glass so the gloves and the eye protection are still mandatory.
Once the glazing is sufficiently removed, prime the area with a thin coat of primer or linseed oil. New glazing will bond to the sealed surface better than against dry wood. This is especially important to those of you doing this job for a window subject to weather.