Better yet Fran; could you do a screen capture (I do this all the time with a preview render) and post it here to see if there's anything obvious we can spot so we can help you? I wouldn't go to the trouble of uninstalling and re-installing before we do this if I were you.
 I've been very happy with Carrara, but it took a little while to come to terms with it. I can assure you that it has a very capable renderer.