If you are getting an error while Windows 10 upgrade which says STOP 0XC000021A or STATUS_SYSTEM_PROCESS_TERMINATED, then its a security issue within the Windows. It is also possible that the system files have an issue, and they have been incorrectly modified. This does not necessarily mean that there is a malware problem, but one of the recently installed application might have modified or corrupted some core files. This guide will help you troubleshoot this problem.
This error occurs when a user-mode subsystem, such as WinLogon or the Client Server Run-Time Subsystem (CSRSS), has been fatally compromised and security can no longer be guaranteed. In response, the operating system switches to kernel mode. Microsoft Windows cannot run without WinLogon or CSRSS. Therefore, this is one of the few cases where the failure of a user-mode service can shut down the system.
1] Uninstall Recently installed Program or Perform a System Restore
A common cause of this problem is some 3rd-party program. Try to identify any new program which you have installed and uninstall it. If you remember the recently installed program, you can try uninstalling them one by one.
However, if the damage is already done, uninstallation will not help. In that case, you should perform a system restore to get back to a stable state of PC.
2] Run System File Checker
Run System File Checker. This will repair corrupted or damaged Windows files. You will need to run this command from elevated CMD i.e. command prompt launched with admin privileges.
3] Rebuild BCD and Fix MBR
Boot Configuration Data (BCD) is a firmware-independent database for boot-time configuration data. To rebuild the BCD or Boot Configuration Data file in Windows type the following command and hit Enter. You will need to use admin elevated command prompt.
To get a new bootloader, type in the following command in the elevated command prompt.
bcdboot c:\windows /s c:
C is your system drive where Windows is installed. If this doesn’t work, you can try repairing the Master Boot Record.
4] Repair hard-disk errors
This may not be 100% full proof solution, but you can run chkdsk in the command prompt to see you don’t have any issue with your hard drive. You could execute the following command:
chkdsk c: /f /r
It is possible that when installing programs on the corrupt part of the disk might cause issues.
5] Contact Microsoft
Let us know if anything here helped you solve the issue.