This is a note from awhile ago, and I’m not sure what the original source for this solution is.
The Problem: The Service Pack 3 installation would start, but was unable to complete successfully.
The Solution involved registering a bunch of .DLL files by running the below commands. A popup appears after running each line.
net stop wuauserv
regsvr32 wuapi.dll
regsvr32 wuweb.dll
regsvr32 atl.dll
regsvr32 wups2.dll
regsvr32 wuaueng1.dll
regsvr32 wups.dll
regsvr32 MSXML3.dll
regsvr32 wucltui.dll
regsvr32 wuaueng.dll
net start wuauserv
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