They're in the same folder as the general preferences file and the keyboard shortcuts file. You can open that folder by clicking the button at the bottom of the Preferences dialog box.
But there is no need to back up the defaults. If you delete the render settings or output module settings file, After Effects will just recreate the defaults.
Many, many thanks for the quick reply, Todd! (And for not making me feel stupid for what was probably a total newb question.). By the way, I have to thank you too for all the OTHER answers you give to people. Most of the time when I'm googling my way to an answer, it's your answer to someone else that ends up solving my problem. I'm a fan :-)
All the best,
Happy to have helped.
(response delayed by a week because I was in the mountains of Brazil)