I found that DNG Converter works fine if I just drag the NEF files from the SD card to the hard drive.
I do have the latest versions of the Nikon software, but I did discover that installing the new version did not delete the older one and that may have caused problems. I have now uninstalled all the Nikon software and maybe I'll get around to reinstalling the latest version of ViewNX2, but since I have only used the software to transfer files from my SD card to my hard drive I may not bother. I use Adobe Bridge for preview.
Thanks to you both for your help 
!