0xC800021F
Could be because of an issue with the Internet cache or corruption in the Datastore folder.
Clear out your temporary internet files etc.
Then I suggest you clear the contents of the Datastore folder as follows:-
- Stop the Automatic Updates Service.
Click Start, Choose Run, In the Run box, type services.msc.
Click OK.
Right-click the Automatic Updates service.
Select Properties.
Under Service status, click Stop.
Click OK. - Delete the Contents of the DataStore folder.
Click Start, Choose Run, In the Run box, type %windir%\SoftwareDistribution
Click OK.
Delete the contents of the DataStore folder - Start the Automatic Updates Service
Click Start, Choose Run, In the Run box, type services.msc.
Click OK.
Right-click the Automatic Updates service.
Select Properties.
Under Service status, click Start.
Click OK.
You'll most likely lose your history - but that's not important.
Once you have done the above, now re-register the MSXML3.dll
To do this - Click Start, select Run and now type: regsvr32 MSXML3.dll
Try Windows Update again