I have written some software to translate files in the outdated NW format to files in the more modern NWC format. The output files are fully accepted by current Noteworthy products, such as version 2.75 (2015) and the free Noteworthy Composer 2 viewer, v2.1 (2010). The software is suitable for use on Windows platforms, including Windows 10.
The help file gives more detail about the conversion, and describes the limits of what is possible within the more restricted modern format.
Download the NW2NWC software with the source code here. You should, of course, not run software from untrusted sources such as this but, if you have any concerns, there are instructions on how to recompile the source after examining the files for trustworthiness.