Microsoft has revoked 28 of its security certificates via Windows Update today as part of their security advisory 278973.
The certificates have been revoked following the Flame malware attack that involved the use of fraudulent Microsoft digital certificates to by pass some security measures in windows to install the malware.
This patch should be available on Windows Update now for Windows XP SP3, Windows Vista, Windows Server 2003, Windows Server 2008, Windows 7.
You should consider applying the update to all windows machines immediately.