Yeah I was able to get a free copy of Poser 5 when I was writing the first book back in 2007 and that is why I was able to use it. Paint.NET is free (allows donations) but to get a 3d character animated you will need a 3d tool. Blender would be a good free option. http://blender.org/
I haven't tried to do any rigging and animation in blender but that would need to be done and then the animation exported in a format that you could extract the images.
The key to any tool is being able to animate something and then render that animation into a format that you can extract the still images and place them into one or more spritesheets.
Hope that helps!