Just thought I'd post the solution if anyone else has had a problem with the 64-bit edition of Windows and installing Nikon software.
After some further experimentation, I found that prior to installing, by selecting the setup program for Picture Project, "right clicking" and choosing the "compatibility" tab, and then selecting "windows XP", PP installed on my 64 bit system, with full functionality, including installation of the appropriate NEF plugins into Photoshop 7.
Thus it appears that the Nikon software works with the 64 bit edition of Windows, and the problem may lie with the installation program.
regards
Terry