Earlier this month, in response to the security breach at DigiNotar, a Dutch Certificate Authority, Microsoft released a Security Advisory (KB 2607712), and an associated patch that removed compromised certificates from the trusted list in Windows. Now Microsoft has updated the Security Advisory, and released a new patch, which includes a cumulative list of all the revoked certificates. This update applies to all supported versions of Windows, but users running Windows XP or Server 2003 are particularly urged to install it. Those systems are most likely to have gotten an incomplete revocation list.
Windows users who have automatic updates enabled should receive the new patch (KB 2616676) automatically; alternatively, it can be obtained from the Microsoft Download Center; you can find download links on this Security Bulletin (2616676) page.
I recommend that you install this update as soon as you conveniently can.