Today Secunia PSI refused to run with the message: "an error occurred while verifying the security certificate". Then I found that IE refused to show https://secunia.com because the certificate was "not issued by a trusted certificate authority". Firefox did not have a problem with that webpage. For some reason, IE did not recognize the "Thawte Server CA" certificate. IE also refused to recognize StartSSL.
This was really weird, because as far as I know, Windows 7 is supposed to automatically update root certificates. Microsoft even explains how the process works in Vista. My first thought was that my firewall was blocking the update, but that was not it. Event log showed event 4100 from CAPI2, which is "Successful auto update retrieval of third-party root certificate". The problem was event 4110: "Failed to add certificate to Third-Party Root Certification Authorities store with error: A certificate chain could not be built to a trusted root authority."
I manually downloaded and installed the latest root certificate update from Windows Update. After that, everything works. I'm just left wondering why I had to deal with this in the first place.