unable to authenticate installer

Bingo. The proxy server is failing to authenticate the installer download. You could ask for the download location to be opened up on the proxy (maybe.. I'm no expert on proxy authentication....)

Or you could get the player from this technote:
http://www.adobe.com/cfusion/knowledgebase/index.cfm?id=tn_14266

Let me know if that does the same thing. And if you have any other threads describing the same problem post them back. I'd like to check this out further when the CS3 launch settles down...

Any chance your machine sits behind a proxy server that requires authentication (name/pwd)??

I am attempting to install the Flash player from the Adobe website to my Windows XP SP2 machine. The installation does not complete and I receive the error message "unable to authenticate installer" from the site. I noticed similar threads with this error message here but none had an answer to the issue.

Please be much more specific about the steps you're using, what you're downloading, and exactly what you're seeing (beyond this message...)