There are several reports on the Internet of issues with Nikon codecs running on 32-bit only. I experience it myself as well.
Nikon's latest version, 1.9, includes support for D3s, and still explicitly states it supports 32-bit Windows 7 only.
http://www.nikonimglib.com/nefcodec/
So, officially, the codec is 32-bit only.
The one I tried last, installed fine on Windows 7 64-bit, but Explorer is 64 bit and the codec 32-bit so Explorer seems not to load the codec. However i've read reports that some of the latest codecs do work on 64-bit. There's also talk of starting the old 32-bit Explorer on the 64-bit box to view the nef files with the codec.
This guy gave some write-up on it. He also talks about a 3rd-party 64-bit codec.
http://robburke.net/2009/02/19/nikon...32-and-64-bit/
Note that CaptureNX and Nikon View, Nikon Transfer all work on Win 7 64-bit, it's only the codec that has issues.