Hi, I haven't overwritten or themed anything on the site.
I have used both Firefox and IE7 to download the link and get the same message on the page (it's in a red box above the list of downloads just to be specific). Sorry, don't know if any of that's relevant but this is beyond my expertise, such as it is!

You're right though, it does seem to be failing and passing some kind of authentication at the same time.

Just to clarify, I'm using public files method. my temporary directory is set at /tmp in the httpdocs folder, the downloads are served from my private folder alongside the webroot. Is this the recommended setup?