If you are still getting the Windows Update Error 646 let’s do some more work. We are going to do a system restore on your system. This will take your system back to a time before you ran the Windows update.
Reboot your computer and press F8 as it is booting up. This will give you the option to put your computer into Safe Mode.
Type restore into the Start menu search box. Click on System Restore at the top of the Start menu.
The Restore system files and settings dialog box opens. You have the option to choose recommended restore, which will undo the most recent update, driver or software install, or choose a different restore point.
If you choose the different restore point option make a choice from the list of restore points.
Choose the restore point and confirm. Restart your computer and it will restore your system to the time you chose.