Securing downloads with iis

Hi

I saw a topic ages ago on a site about how to secure
downloads on a web page, but I can not remember where, it
involved loging in to a site by any means, then if you
loged in successfully (asp pages) the site would start a
download, but it could time out, and the directory kept
changing e.t.c. basically it is secure

if anyone knows of anything that is similar please tell
me as i really need to secure downloads on my web site

Thanks

William Hilsum