You might try comparing the results from using the scanner's software, against copying the negative into Photoshop and converting it there.
As mentioned in several of the discussions on this topic, I have two or three wonderful pictures of my mother with me aged <1. I'd never seen them positive before. The negs were from an old concertina Kodak, and very dirty. I used a lens cleaning technique told me by a microscope shop owner in London. A huff, pause and wipe. It seems that there's just enough water vapor to do the trick without risk to the neg. Erm, you need to convince yourself of this, on ones that you don't care about.
Second link down. Remember, this was taken with a very primitive camera.
http://www.pprune.org/computer-inter...ml#post2604951