Protect PDF Files Via ASP?

I have several PDF and MPG files I would like to provide to users to
download via HTTP. I also have a database of user accounts. I would like to
protect the PDF and MPG files so that users cannot "save target as" or "view
source" to directly link to the files.

My first thought is that I would have to remove anonymous access to these
files and/or their parent folder within IIS. I was thinking that I could
then create a Windows account called something like WebUsers and give it
access to that folder. I'm hoping to write some ASP code that authenticates
my users against my own database and, if successful, logs them into IIS via
the WebUsers account (so that all my users share the same account).

My problem is I cannot find an code or method or object to do this. Is there
some simple function that I can use to pass a username, pw, and domain to
IIS to authenticate the user that would then carry through for them to be
able to download non-ASP (PDF, etc.) content?