20Microsoft-Windows-WindowsUpdateClientWindows Update AgentInstallation Failure: Windows failed to install the following update with error 0x80070643: Security Update for SQL Server 2008 R2 (KB2494088).
I had this issue on Windows 7 workstation that only had the SQL Management Studio installed. For some reason, Windows would try to install the update on every shut down, but always fail.
The only way I was able to resolve it was to do a repair on the SQL Server 2008 R2 installation. While the repair didn't find anything, and resulted in the setup crashing as well, a reboot later Windows no longer prompted me to install the update and KB2494088 is now registered as installed on the computer.