1. Open the Start menu and click on the 'Shut Down' button.
2. Choose the 'Restart' button from the drop-down menu and click 'OK.' The computer will now safely shut down and begin the rebooting process.

3. Press the 'F8' key on your keyboard when prompted during the boot process.
4. Use the up-and-down arrow keys to highlight the 'Last Known Good Configuration' option. Then hit 'Enter.'
5. Use the up-and-down arrow keys to highlight the operating system version that you wish to boot up to, and then hit 'Enter' again. The system will now restart to the last working version of your operating system, which will eliminate the C000021a fatal error.