I found an old .NRG file, which is a proprietary CD image file format from Nero. I wasn't having any luck with tools to convert this file to an .ISO (or even extract the files).
I ended up finding a ImgBurn, a free software tool that was able to read the .NRG file and burn it to CD. Then, for safe keeping, I used ImgBurn to read the new CD and save the contents in the more future-proof .ISO format.