1. Close the error message window concerning the C000021 fatal system error.
2. Open the Start menu and click on 'Shut Down.'

3. Select 'Restart' from the list of options and click the 'OK' button.
4. Hit the 'F8' key on your keyboard when prompted to on the reboot screen.
5. Use the arrow keys on your keyboard to scroll down and select the 'Last Known Good Configuration' option from the reboot screen, and then hit 'Enter.'
6. Use the arrow keys again to select the operating system that you wish to reboot your computer with, and hit 'Enter' again. The Windows operating system will restart and restore to the last working version, which will eliminate the C000021 fatal error alert.