0xC800021F

This error may happen if the information stored on "Windows\SoftwareDistribution\Datastore" gets corrupted or damaged. To solve it, it is enough to type in these commands at the command line, Start > Run and type CMD.exe; hit ENTER key after each one:

regsvr32 MSXML3.dll /s
net stop wuauserv
cd /d %windir%\SoftwareDistribution
rd /s /q DataStore
net start wuauserv
exit


These commands will register the related libraries and delete the folder. This one will renew when restarting Windows. If you do not want to type these commands you can download this CMD file which will do the same thing. You only need to download it and click twice on its icon.