# Boot loop, BSOD c000021a, verification of a knowndll failed



## Axeman72

*Boot loop problem. BSOD c000021a Fatal System Error. Please help* I have a custom built computer. Windows 7 64 bit, 8 Gigs Ram. As of two days ago I can't boot into windows. I get to the colored windows logo then it restarts. Same thing in safe mode. I tried a system repair and it fails each time. Tried last known configuration and that didn't work either. I'm running out of ideas. The BSOD reads:

Stop: c000021a {fatal system error}
The verification of a knowndll failed. system process terminated unexpectedly with a status of 0x000012f (0x0090d890 0x00000000).
The system has been shut down.


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## Blade_Jones

An error check of the drive might repair a bad Windows file. 
Any .dmp files located in C:Windows/Minidump ? If so post the most recent one if it matches the date and time of one of the crashes. 
Run Drive Fitness Test.


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## Axeman72

Thank you for the reply. Is there a problem running any of those tools on a SSD drive? I already ran one check on the drive and it showed no errors. I tested the RAM, no errors, and the video card, no errors. I will wait for your response.


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## VirGnarus

The verification process for the dll failed because for some reason a responsible file was in the process of being deleted at the time (hence status 0x12f). That's pretty odd...

Anyways, do you have the Windows 7 installation cd? If so, slap that in and boot up with it, then go into the Windows Recovery Environment by selecting the option to repair your Windows at startup, then after it is done the startup repair, there's a link that says "_view advanced options for system recovery and support_". That'll open up a list of opens. Go to the command prompt one, then type _chkdsk /r_. Let that run (it may take a while). Hopefully that might fix things up a bit.


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## Axeman72

Thank you for replying. I do have the Windows 7 CD. I will try your suggestion and get back to you.


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## Axeman72

I completed the chkdsk/r scan and I still get a BSOD that reads very similar:

Stop: c000021a {fatal system error}
The verification of a knowndll failed. system process terminated unexpectedly with a status of 0x000012f (0x0008c550 0x00000000).
The system has been shut down. 

After the scan it still failed to start windows, no safe mode, and would fail when I tried to repair. What should I try next?


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## VirGnarus

So the chkdsk /r completed successfully, or was there an error? Also, from the same recovery environment, there's an option to do a System Restore. Are you able to perform that, or does it not work regardless?

If this is all happening in safe mode as well, most likely you either have a hard drive issue or some nasty malware got into your system. You can use Seatools in the Hard Drive Diagnostic section of the UBCD (Ultimate Boot CD) in order to determine if this is a hard drive problem. If you suspect possible malware infection, I'm afraid it is not in my jurisdiction to help you, and you will have to post a thread in our Virus/Trojan/Spyware subforum so our security analysts can assist you properly.

There's also another possibility of corruption of Windows that was unintentional (doesn't seem like it), in which case your only option would be to install Windows 7 again over the existing copy and reinstall all your programs and whatnot. Obviously, it ain't pretty.


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## Axeman72

Chkdsk /r did not show any errors. Since the beginning, system recovery says it cannot fix my installation of windows. I still can't get into safe mode. I will try the Hard Drive Diagnostics scan tomorrow. I'm still not sure if it is hardware or malware related. I will let you know the results of the scan tomorrow. Thank you.


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## Blade_Jones

I like to run a drive fitness test from the get go because it's time wasted if you're repairing the OS on a failing drive.


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## Axeman72

OK, I ran the hard drive test and there were no errors. I ran a CPU stress test, no errors. I ran another memory test and no errors. I took out my graphics card, and used the on-board video, no change. I swapped memory, no change. I looked at the motherboard and didn't see any bulging capacitors or anything. What am I missing? Should I just re-install the OS. I was trying to avoid that, mainly because I would like to know what the problem is.


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## VirGnarus

The thing is there's simply a form of corruption that's taken place in the Windows environment in that a crucial system process got buggered up and Windows will refuse to start without it running properly (due to this weird "file deleting" issue). This can be caused by a 3rd-party driver for some software. Though what concerns me most is that you have it doing the same in safe mode, which leaves 3rd-party drivers out as a possibility (unless it's malware).

Unfortunately unless you can use Parted Magic from the UBCD to find either minidumps (in _/Windows/Minidump_ directory) or the kernel dump (even better; called MEMORY.DMP located in Windows folder), the only other way to diagnose this would be to do live kernel debugging, which just ain't gonna happen. If chkdsk /r ran completely but did not find anything, your only other option at the moment is to use the recovery environment to attempt a System Restore. 

If that even fails, looks like you'll have to do an inplace installation of Windows 7 over your existing, then move whatever's in the .old folder to their respective places. That should preserve the majority of what you have. You can install a 2nd copy of Windows 7 and boot into that and see if that runs to determine if it's a hardware issue or not. If it boots up fine, your drive is OK. If that also has problems, you got hardware issues (drive problems if it's the same error).


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## Axeman72

Well, I'm pretty confident that it is not hardware related. I guess only time will tell. There was not any minidump or memory.dmp files that we could look at. System restore did not work either. So, all that was left for me to do was reinstall windows. I'm in the process of updating the new installation. I feel bad because I have not had to reinstall in a long time. I can usually figure these things out, sometimes with the help of people like you two. I thank you for your time and effort you have given me.


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## Blade_Jones

This is probably not causing your problem but one last thing that you might do if you've got an Ultimate Boot CD, ERD Commander, or a BART PE CD (requires special instructions to load the registry) is to check the registry to confirm that these values are correct as follows....
HKLM/Microsoft/WindowsNT/CurrentVersion/Winlogon/Shell = Explorer.exe
HKLM/Microsoft/WindowsNT/CurrentVersion/Winlogon/Userinit = C:\Windows\system32\userinit.exe,

Usually it's viruses that change these values and cause the logon failure loop.


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## VirGnarus

Sorry for not being able to assist further. Unfortunately, without raw data like crashdumps, we can only go far as to speculate possible cause, given what you have explained to us. Usually crashdumps fail to be produced when one or more of the following occurs:

1. The crash occurred before the appropriate disk drivers have been loaded, preventing Windows from writing the crashdump to disk.
2. The paging file has been exhausted, too small, or is not located on the primary Windows partition.
3. The disk was unresponsive or generated an error when attempting to create the crashdump.
4. Windows settings were manually set to not generate crashdumps.

It's rather late to mention these, and redundant, but that's typically how it goes when crashdumps fail to produce. Fatal system errors like this often occur before the proper drivers and modules are loaded to produce crashdumps, and with a disk I/O related error like this, it leads suspicion towards the drive or drive controller drivers, or the actual hardware itself. Again though, we can only speculate without raw data to read.


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