If you have this particular error message (STOP 0xC000021A Fatal Error) on your Windows XP or Vista computer, then most probably your computer will not start or boot up. Also, most of the boot options will not work anymore. I know you are probably hoping to fix the problem without reinstalling your operating system, however I recommend that you change your focus into How to recover your data, as you will be exerting a lot of effort in trying to fix this and most probably you will end up reinstalling/reformatting your system anyway.
If you still want to fix this issue then you can refer to this Microsoft article regarding how to troubleshoot a “STOP 0xC000021A” error.
If you want to recover you data, then you can follow the steps below:
You need another computer so that you can remove your harddive and make it as a slave drive on the other computer. By doing this you will be able to access all data on the drive and back it up from there. After backing up your files, you can go ahead an install your Windows XP or Vista operating system. I really recommend this step for restoring all your data as it allows you to completely recoving everything! If you do not really care about restoring your files, go ahead and install your operating system using your windows XP disc. It should take less than 20 minutes. Good luck!
