Protected Download Of Files...

I am creating a series of downloadable articles, career
tools, spreadsheets, etc. However, some of the content is
for "registered" guest. The registrants are stored in a
database and logon.

I want to ensure that someone registered cannot simply
pass someone the path to the files for download. However,
I am not creating an actual system user name for the
folders where the various documents exist.

I don't want someone to logon, retrieve the path to a
file, and be able to send that path or return to that path
without having to logon.

How do I accomplish this?

Consider the following:
serverroot/registeredcontentfolders would be protected by
a single logon name - not accessible via anonymous
access. User's logon to site and I auth them with their
user name but on the server side, auth with the single
account I am establishing for access to the protected
folders.