Previously, I had installed the TG2-64 from the Administrator's account and checked "Used by Everyone" instead of for use only by that one account. This time, after installation was complete, when I right-clicked a TGD file and selected "Open With", the Terragen 2 program showed up as an option for the first time. Apparently I am the only one who is installing the programs in my Administrator account and selecting it to be used by everyone. I suspect that I just ran into a deficiency in the TG2 installer. However, I still can't explain why W-7 would not let me associate the file extension with the program that I was able to navigate to and select.