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:-

1. 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.
2.  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
3. 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

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