There are number of free programs out there which should do the job (but as I don't have a Nikon I can't test this properly).
I use IrfanView to browse images (it can also do a bit of editing as well as conversion) and it advertises NEF compatibility. It is available here
and don't forget to download and install the plug-ins available here
EDIT: I have just tested IrfanView and it understands D80 NEF files. You can set up a Windows file association. If you want to use the File/Open menu then you need to select "DCR/DNG/EFF/MRW/NEF/ORF/PEF/RAF/SRF/X3F - Digital Camera RAW Format
" in the "Files of type
" listbox before you can actually see the NEF files listed. They aren't there when you select "All supported Graphics files
" in that listbox which appears to be a bug. Once open you can convert them to JPEGs as you desire.
FastStone Image Viewer
is apparently another option if you don't like IrfanView.