See: "Error message when you try to use the Windows Update or Microsoft Update Web sites to install updates: "0x800B0100""
http://support.microsoft.com/kb/956702
Meanwhile, for information about Security updates, visit the Microsoft
Virus Solution and Security Center
for resources and tools to keep your PC safe and healthy. If you are having issues with installing the update itself, visit
Support for Microsoft Update
for resources and tools to keep your PC updated with the latest updates.
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Vincenzo Di Russo - Microsoft MVP Windows Internet Explorer, Windows Desktop Experience & Security - Since 2003