One of the complainers on Google AdSense Help says:
Windows XP visitors using IE 8 on several of my AdSense sites see an error message "Internet Explorer has blocked this website from displaying content with security certificate errors."
Without AdSense loaded there is no error. Turns out you have to update the root certificates for your computer at windows update manually if you have an XP computer. Windows doesn’t automatically update these certificates for XP users.
It appears to be a problem with the internet explorer version and only occurring on pages with AdSense.
Although the issue hasn’t been sorted out yet, and there is no idea why it is happening?