Sharing this because it has taken me nearly 3 days and much research and frustration [tried every suggestion offered] before I finally found the fix for my problem. I wont go into the details of why .. but Windows Update had ceased to work [I had to reload Windows Installer] .. and it would download but failed the installation .. the error message was not helpful .. just 'failed' and that was it, but it kept making me reboot to 'finish' the installation and clear the message so I could try again. It took me a while I finally identified that the error was caused by the update program being unable to write to the necessary registry keys [Access Denied]so, not necessarily only the update program .. this problem could come from any program which needs to change system keys and might apply to other applications which report this error. Came eventually to this article: "http://support.microsoft.com/kb/313222/" which describes how to reset the Registry Key permissions to the installation defaults .. THIS WORKED .. Windows Installer installed okay - Updates now working.