Sorry to sound like a total noob, but where exactly do I find the root folder on my N73`s memory card?

At the top. The "root folder" is the topmost folder.

If you use the built-in File manager ("File mgr.") app in the phone menu, it will start showing internal phone memory (from the device's operating system point of view, this is the C-drive).

Now, press the "joystick" key to the right, to switch from internal memory to the memory card (the E-drive), and you're now shown the "root". There you'll see subfolders (subdirectories) like "Data", "Images", "Other", "Sound clips" and "Video clips".

What about when i`ve got card in card reader in my PC, or does the same apply?