Out Of Band Windows Update

Microsoft has issued an important out of band update for XP, 2K3, Vista, 2K8, and 7.
Simply run Windows Update to install it.
Microsoft Security Advisory (2607712)

Microsoft Security Advisory (2607712)
Fraudulent Digital Certificates Could Allow Spoofing
Published: August 29, 2011 | Updated: September 06, 2011
Version: 3.0

Executive Summary
Microsoft is aware of active attacks using at least one fraudulent digital certificate issued by DigiNotar, a certification authority present in the Trusted Root Certification Authorities Store. A fraudulent certificate could be used to spoof content, perform phishing attacks, or perform man-in-the-middle attacks against all Web browser users including users of Internet Explorer. While this is not a vulnerability in a Microsoft product, this issue affects all supported releases of Microsoft Windows.

Microsoft is continuing to investigate this issue. Based on preliminary investigation, Microsoft is providing an update for all supported releases of Microsoft Windows that revokes the trust of the following DigiNotar root certificates by placing them into the Microsoft Untrusted Certificate Store:
• DigiNotar Root CA
• DigiNotar Root CA G2
• DigiNotar PKIoverheid CA Overheid
• DigiNotar PKIoverheid CA Organisatie – G2
• DigiNotar PKIoverheid CA Overheid en Bedrijven