How do I protect downloadable files?

Hello, I have wrtten a PHP based web site with a MySql backend and now I want to password protect downloadable files. I have logon and session handling taken care of but I can't figure out how to only allowed those who are currently logged in and above a certain security level to access the downloadable content and prevent bookmarking of the file location for redownloading. Currently I have a .htaccess file to protect the files but then you need to enter a User ID and password a second time. I would prefer a single signon solution. I have considered copying the files to a temporary area each time someone wants to download it and then erase it when the session is killed but these files can be large (20-100 mb) and I would rather not do all of that copying if possible. Creating unique symlinks would be easier but my development machine is Windows and my server is FreeBSD and I can't create file links under Windows. Plus, my FreeBSD server is not near me so remote development is difficult. Thanks to anyone with any ideas, Andrew