For the past 8 months or so, all of the Microsoft Office Security Updates that have come through the Windows Update system have failed to install on my home WinXP machine. I couldn't find any solution to the problem in the Microsoft Knowledge Base, so I've been checking the "don't ask me to install these updates again" button on Windows Update and forgetting about the security updates.
Now that the Office security issues are started to stack up, I decided to find the problem. Since the error messages from Windows Update are totally cryptic, I decided to use the Office Update system on the Microsoft Office web site. Even though it appears to use a different update system, the updates still failed to install. However, the web site helpfully listed about 10 possible reasons for failure, with the last reason in the list mentioning something about a missing registry key.
Sure enough, the Office Update web site links to Microsoft Knowledge Base Article 884298, which describes a single registry key string that simply contains the path to the Office install CD location.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\ Delivery\SourceEngine\Downloads\ 90000409-6000-11D3-8CFE-0150048383C9\Sources\ 90110409-6000-11D3-8CFE-0150048383C9\Path
My system didn't have this key, which caused the Office updates to fail. After I added the key to my registry, the updates succeeded. As a matter of fact, I also (inadvertently) found out that if you add the key and then set the key value to a random path, the updates still work successfully.
I have no idea why the key was missing on my system, but you'd think with all of the sophistication of the Windows Update system, they could have just fixed this for me automatically, rather than punt with a error.