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I'm experimenting with a Windows 8 installation which is on a separate SSD. My actual Windows 7 installation I'm working with is on my old HDD.

While Windows 8 was in sleep mode I swapped the hard disks and put in the Windows 7 HDD (I thought the computer was off). When I started the computer, Windows 8 started back up to the login screen – then it was stuck and some seconds later the computer rebooted.

Now the Windows 7 Installation is damaged. When I boot, after the Windows 7 startup logo appears, a bluescreen shows up for few seconds stating:

STOP: c000021a {Fatal System Error}
The verification of KnownDLL failed. System process terminated unexpectedly with a status of 0xc000012f (0x00f0bb90 0x00000000).
The system has been shut down.

and then the computer reboots. The same happens in safe mode. 'Windows startup repair' cannot repair the issue.

Any idea what could have happened exactly and/or how to repair this Windows 7 Installation?

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I would use the repair console. This isn't Windows 8 fault. You need to verify the hdd wasn't damaged because of your actions. – Ramhound Oct 25 '12 at 11:08
I can use the repair console, but the automatic repair feature doesn't work and there are no restore points. So I don't really know what to check for – Mat Oct 25 '12 at 11:33

I've gone down the route of checking online for issues relating to that error message, as since Windows 8 has just been released I doubt people have had the same problem. Possibly the same error, but not the same cause.

Something you could try is putting the HDD in another PC so you can look at the contents. Search in the windows folder for anything modified after you booted it up with the error. Possibly (some of) those files are ones that might need replacing.

Searching online hasn't revealed a definite fix, so I only offer a few things to try.

According to this page:

you could try restoring to last known good configuration from Advanced options in the F8 boot menu.

There are also various posts on forums such as this one indicating you should replace a few files in the windows folder with known good ones from a working installation.

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The best method is to put windows 7 disk back in as it was- remove the SSD and any other poterntial ones.

Put in a Windows 7 CD and boot into it. Use the "Repair your computer" link

enter image description here

Use the Start up repair link

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You need to put some trust into this procedure because allot of people slag it of and do not fully udnerstand what it does. You just need to know to try it up to 3 times... yes! 3 Times! If it cannot repair after 3 times then something serious has happened that cannot be reverted... eg overwrote some sytem files like registry.

The repair tries to repair corrupt dll's, copy them in if they missing, scan the registry for corruption(does not fix it though), tries to use windows restore to fix corruption(such as partial registry issues, DLL's, etc) Each pass might fix something but something else might be broken down the line - thats why you have to do a repair up to 3 times.. Even it it fails main booting- try safe mode- If you can get into safe mode then try reinstalling drivers(GFX,RAID,etc), removing antivirus or any other system depended 3rd party..

I hope it helps you.. It has done for me many times!

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