Turn off the Auto-Update on your Nano-Render preview. Only click on it when you want to see an update. Having it update a slow render every time you move something sounds like part of the problem. If your render is that slow, it sounds like you are using lots of Trees, Transparencies, and/or Reflections. Or you just have so many objects in the scene that you just don't have enough RAM to get through the whole render. What kind of machine and OS are you using?