The solution is VSO Image Resizer. There are a few other options out there but the reason I think this is the best solution is that you have a massive amount of control over your image conversion including setting resolution, quality, watermark, destination format, destination naming convention and on top of all that, you can do bulk conversions right from your Windows context menu. (select all that you want to convert, right click and select VSO Image Resize> )

Depending on the size of your initial NEF image (usually quite large) and how many images you convert, the conversion can take some time but the program will actually warn you when it thinks the conversion will take an exceptional amount of time so you will at least be warned.

This works really well for those who aren’t geeky enough to have a Linux install

You can download VSO Image Resizer here: http://www.vso-software.fr/products/image_resizer/