WinUpdate: Error C0000121 applying update operation 127704 of 127761 (\Regis...)

Because this machine is  dedicated to special tasks, there is no AV software installed.

The error occurs at the very end of the updates at start-up, so it seems like a file clean-up failure, based on what you posted.

However, searching under the "0xC0000121" instead of "Error C0000121", I see a few comments suggesting the use of third party software to run to fix registry errors.  Given that the error ended in "(\Regis...)", with the rest of that unviewable, it maybe be a missing registry entry or something?

The machine is dual-boot, because I need XP for older hardware/software combos (still), but I don't suppose booting into the old op sys will help if it is a registry problem, although I could edit files easily.

The recovery menu under 8.1 offers an option to go to a restore point.  I tried that.  However, the restore point failed, reporting that some Silverlight file had been removed from the package.

So, I may be between a rock and a hard place with this one, right?

Win7 and 8.1 have been very stable for me, so I'm not exceptionally familiar with the recovery tools.

Although I''ve kept sys images, I do not for this machine.

The "Automatic Repair" menu that comes up on reboot after the update crash gives a troubleshoot option.  There you can "refresh" [?], "reset" [?], use DaRT, or Advanced Options. 

Under advanced, I tried "Startup Repair".  It reported failure without cause, but did do a disk check, so it is not a corrupted file, one supposes. 

Under "Startup Settings", I chose 'restart', which gave a menu including 'safe mode' on restart.  However, the computer launched into the update process rather than go to a Win 8.1 login screen and that resulted in the same dead-end, the 0xc0000121.  I'm not sure what I'd do under safe mode anyway.  Run third-party utilities?

I tried to do the System Restore with an eye toward maybe just replacing the file that it said was missing, but the restore point I used before is now missing.

So, I'm ready to wipe the Win8.1 install and start again, to save more time troubleshooting it.  It's a pain, but there isn't that much of a program/client payload running under it. 

What's the best way to do that completely safely.  That's the question, because I absolutely cannot interfere with the old XP install, which has old-but-good drivers and so forth that would be a pain to track down again.

comments suggesting the use of third party software to run to fix registry errors. 

I wish I knew whether any of those were at all reliable.  I more fear that that they may be opportunistic and trying to peddle something.

machine is dual-boot

Then you can use it to look for diagnostics.  Another poster is currently having a problem with reg.exe being out of date and otherwise appearing to be corrupt.

https://social.technet.microsoft.com/Forums/en-US/f856b1d8-d3e6-4b9f-8ec0-74b961faf781/cbslog-regexe-do-not-match-actual-file-and-corrupted?forum=w8itproappcompat#3eff7840-d459-410a-9d9b-3c435f1118bf

 restore point failed, reporting that some Silverlight file had been removed from the package. 

What about booting in safe mode?

Error C0000121

Do you have an AV running then?  It may be interfering.

https://forum.avast.com/index.php?topic=56562.5;wap2

(BING search for

    0xC0000121

)

<quote>

{An attempt has been made to remove a file or directory that cannot be deleted.},

</quote>

As it is, the system hangs shortly after this error, so that I cannot boot into the Op Sys.

Try booting in safe mode first.