well it sounds like that should not happen at all. It sounds like there might be something seriously wrong with your software. Carrara is quite a good renderer and only renders blurry stuff if you tell it to, so if your image is smudged and foggy, it is possible the software is corrupted and not rendering properly.

If there is nothing in your scene settings, lighting and objects that would cause this, I would uninstall, do a system check and then reinstall. If the problem is still there after that, it would be a good idea to post screen images of your scene and the render. That would give some idea of what you are getting.