Microsoft Update for compromised CAs [Update]
Microsoft published a new Microsoft Update for Certificates.
After several CA hacks during the last days, this update should be installed asap.
http://support.microsoft.com/kb/2607712
This update is currently not available on Microsoft Update or Windows Update, so you need to perform the update manually.
[Update 1]
For Windows 7 and Vista this update is now available under Windows Update.
[Update 2]
Now also available via Windows Update on WinXP.