Plug in is like a mini program that helps you do certain modifications faster . Example, if you want to convert lots of colored pictures into black and white, you might download a photoshop plug in that can do that for you or just use a macro.
When you open up photoshop, it detects and recognizes all the plug ins installed.
Nikon plug in could have been installed if you own a writing tablet, like waccom and such or as a package with photoshop.
You can go to control panel, add remove programs and remove the plug in there or disable it in photoshop.
Or go to the Adobe Photoshop Plug-ins folder
C:\Program Files\Adobe\Adobe Photoshop CS3\Plug-Ins\File Formats\ and then select and remove the Nikon NEF Plugin LE.8bi file.