Sorry, I am a complete beginner who's just installed Linux Mint 17 with the Cinnamon desktop on a computer with an Athlon 64 3000 on an Asus GA-K8NF-9-F2 motherboard with an Nvdia nforce4-4x series video card, 3 gb ram, most all, except the ram, about 8 years old.

I don't have the faintest idea of how to get out of the fallback cycle. I have installed the 64 bit system from the USB drive. It worked fine for a day, I set the printer (a Samsung ML-3051ND, attached to the computer by USB, works fine except for duplex printing, but that's a different story). I printed some texts for work with Libre Writer, and many of the dialogue boxes were illegible, very garbled. So I went to Hardware Driver app, and since I had already set the video driver to the recommended Nvidia setting, I applied, not reverted to the X-nouveau driver.

Then I opened Firefox, went to youtube, and started looking for certain videos useful for my students. While running one of them the system froze up completely.

I waited, the mouse pointer would not move, none of the keys seemed alive and finally I rebooted only to find the computer in Fallback mode. I have tried to follow the discussions and instructions found through google (I got Firefox to work on the system by opening it through the Help dialogue on Computer or Home.

I haven't the faintest idea how to open a terminal window, how to run a program from the terminal once I open it, nor how to edit, or find, a configuration file. In other words, I am lost, forgot everything I ever knew about unix and command lines...

Any help forthcoming or is mine a hopeless case?

This should help:

https://help.ubuntu.com/community/Bi...erHowto/Nvidia

Your hardware is not the best for trying to run Cinnamon.
You would get better results with Mint 17 Mate or xfce. They are less demanding of hardware resources.