“ code : 646 windows update encountered an unknown error”

windows update with error code “ code : 646 windows update encountered an unknown error” on windows vista or windows 7

Then please follow the below steps :

Step 1: Turn off UAC (use MSCONFIG)

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1. Click Start. Type MSCONFIG in the Start Search bar and press Enter.

Note: If UAC window prompts for permission to continue, please click Continue.

2. Click Tools.

3. Highlight Disable UAC, click Launch.

4. Restart the computer and test the issue again.

Note: If you turn on UAC, please launch Enable UAC

If the steps do not work, let's refer to the following steps to manually download and install the updates.
Manually download and install updates
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or ,

1.Click Start, and then in the Start Search box, type regedit.



2. In the Programs list, right-click Regedit.exe, and then click Run as administrator. If you are prompted for an administrator password or for confirmation, type the password, or click Continue.



3. Locate and then click the following registry subkey:



HKEY_USERS\.DEFAULT\Software\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders



4. In the details pane of Registry Editor, locate AppData. The value of this key should be %USERPROFILE%\AppData\Roaming.



5. If the value is not set to %USERPROFILE%\AppData\Roaming, right-click AppData in the details pane, and then click Modify.



6. Enter %USERPROFILE%\AppData\Roaming into the Value data box, and then click OK.



7. Close Registry Editor.



8. Try to install the update again. If the problem persists, repeat these steps but use the following registry subkeys instead of the subkey that is specified in step 3:



HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders

HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders



Now perform the windows update.