You boot your PC or Laptop up to a Blue Screen "Fatal System Error". c00021a
I will show you what I have done to fix this issue, it works on most systems. It works on Windows XP to Windows 7.
Note this may fix your issue or not, please use at your own risk.
Step 1: First Step
The first step is to use the F8 key at boot, once you get to the Boot Menu, choose Last Good Known.
This will fix the problems some times, if it didn't fix the issue, see other steps.
Note to get the Boot Menu up, Power on your PC and press the F8 key until the menu pops up
Step 2: Boot to External Media
Now if Last Good Known didn't work, we will have to boot your PC to some external Media, I am using Hiren's Disk.
They are also some Linux Boot Disks that you can use too.
Step 3: Navigate
From the external boot disk, navigate to %SYSTEMROOT%\system32\config\RegBack folder
On most systems this will be your c:\Windows\system32\config\regback
In this folder is a backup of your Registry
Step 4: Copy
From the RegBack folder, copy the following files:
Now go back one folder, the config folder and paste these files into it, overwriting the other files.
Now Reboot your PC, it should boot up into Windows, if not you have other problems.
I hope this will fix your Blue Screen, I have fix a lot of PC's and Laptop with this particular Blue Screen