We cannot know because you did not provide any more info about your system. This could simply have something to do with recent security updates on your system in the course of which certain certificates have been revoked because they were compromised and e.g. your Windows may wrongly also have blacklisted Adobe certificates. You might wann do some research based on that info...
What do you need to know? I have Windows XP and Adobe Reader 7 currently, although I removed the program and now cant even get that one back! I went over to my laptop and the install program ran fine. On my desktop machine it says certificate authentication failed!
Not sure what settings I might need to change??
Turn off Zone Alarm? Turn off firewall?
Actually I found the solution. It was simple and took minutes to resolve!!! I flushed Adobe's Acrobat Reader down the toilet by uninstalling it, because I could not get the error message to stop, and the damn free reader wouldn't install. I then downloaded FOXIT PDF reader, and man it installed with no problems and works like a charm! I am so happy. Thanks for trying. I just wanted to be able to read PDF files so I could get on with my life, not spend hours farting around with Adobe trying to understand why it won't install. Keep up the good work Adobe and your share prices are sure to plummet!