"These files are old enough that they go back to Win 7 era"

Of course they do, because Microsoft Security Essentials ended with Windows 7. In Windows 8, it was renamed Defender, and for Windows 10, it still is Defender.

.cfg filetype (or "extension") is a configuration file.

"in one of our folders"

Since I don't know your system, it's up to you to decide if you need to take any action. Example: If Lacerte crashed back then, the .cfg files might be copies of the real .cfg files, but these were logged into that folder you are examining. You might be able to delete that entire folder, in this example.