The options provided below might help you solve the problem.  
Suggestion 1: Restore two registry branches.
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Let's repair the following two registry keys:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\wuauserv 
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\Root\LEGACY_WUAUSERV  
1. Click "Start", click "Run", type: "Inf" (without quotations) and press  "Enter". A folder will open. 
2. Locate the file "au.inf", (you can type au to locate the file), right  click it and choose "Install". 
Note: The file extension ".inf" may be hidden; we may only see the file named  "au". 
3. The setup process will try to restore the registry branches automatically.  
However, you may not receive any notification. 
If some files cannot be located  automatically, we will be prompted to select the installation source. Please  click "browse", and go to this location: 
C:\Windows\ServicePackFiles\i386 
If this folder is unavailable, please choose the folder  "C:\Windows\System32". 
4. Please restart the computer.
Now try Update to see if the issue has been resolved.