Irfanview Doesn't Open or Convert NEFs (RAW Nikon files)

I wanted to convert a bunch of NEF (Nikon camera RAW files) to JPG with IrfanView (I remembered it had such an option), but it wouldn't work—it started the batch but just gave errors like:

Error! Can't load [file name here]

Without any further explanation.

When I tried to just open NEF files, it gave another error:

Can't load PlugIn: "FORMATS.DLL"

This is despite the fact that I had formats.dll in the Plugins directory as instructed.