Code:
$ pwd
/opt/calibre
$ find resources/fonts/
resources/fonts/
resources/fonts/liberation
resources/fonts/liberation/LiberationSans-Italic.ttf
resources/fonts/liberation/LiberationMono-Bold.ttf
resources/fonts/liberation/LiberationSans-Regular.ttf
resources/fonts/liberation/LiberationSerif-Regular.ttf
resources/fonts/liberation/LiberationSans-BoldItalic.ttf
resources/fonts/liberation/LiberationMono-BoldItalic.ttf
resources/fonts/liberation/LiberationMono-Italic.ttf
resources/fonts/liberation/LiberationSans-Bold.ttf
resources/fonts/liberation/LiberationSerif-Italic.ttf
resources/fonts/liberation/LiberationSerif-BoldItalic.ttf
resources/fonts/liberation/LiberationMono-Regular.ttf
resources/fonts/liberation/LiberationSerif-Bold.ttf
resources/fonts/calibreSymbols.otf
That's a standard calibre installation on Linux, I assume it's roughly similar on Windows.
PS: User settings are in ~/.config/calibre on Linux, so I guess that's where user-installed fonts would end up.