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To unblock a software publisher:
1. In Internet Explorer, click the Tools button, and then click Internet Options.
2. Go to the Content tab, Look under the Certificates section.
3. Click on Publishers button.
4. Select the Untrusted Publishers tab.
5. Select the Publisher you wish to unblock and click the Remove button.
Alternatively, you can download the complete Flash package from the Adobe website here: http://www.adobe.com/special/products/flashplayer/fp_distribution3.html?PID=4172469
Moving this discussion to the Installing Flash Player forum.
I just got a chance to try... and I do not see a publisher that looks like adobe to unblock.
So I tried the alternative to download the complete Flash package and continue to get the error.
Can you tell me the exact error that you receive? What version of Windows are you running and what browser are you trying to install Flash Player on?
HOST Certificate Authentication failed
I've tried both IE 8 and Firefox.
I'm on Windows XP Home Edition version 5.1 with updates.
Sorry, I've followed all kinds of prompts/suggestions I've found and it won't work!