Unexpected shutdowns with BSOD c000021a on startup

Yup
Do you remember the stop code?

Okay so time for waiting till your computer crashes again..

If you want we can enable Driver Verifier

In the meantime fill in your specs
System Info - See Your System Specs

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1. Click on User CP on the top right of this page
2. Click on Edit System Spec
3. Fill in your computer specs
   System Info - See Your System Specs

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Edit: Did you look for an update on your BIOS version?

Code:

BIOS Version/Date	Phoenix Technologies LTD V1.04, 10/22/2009

:)

You'll be able to use your computer. No problem.

Lets enable driver verifier to rule out buggy drivers.

Driver Verifier

I'd suggest that you first backup your data and then make sure you've got access to another computer so you can contact us if problems arise. Then make a System Restore point (so you can restore the system using the Vista/Windows 7 Startup Repair feature).

In Windows 7 you can make a Startup Repair disk by going to Start....All Programs...Maintenance...Create a System Repair Disc - with Windows Vista you'll have to use your installation disk or the "Repair your computer" option at the top of the Safe Mode menu .

Then, here's the procedure:
- Go to Start and type in verifier and press Enter
- Select Create custom settings (for code developers) and click Next
- Select Select individual settings from a full list and click Next
- Select everything EXCEPT FOR Low Resource Simulation and click Next
- Select Select driver names from a list and click Next
Then select all drivers NOT provided by Microsoft and click Next
- Select Finish on the next page.

Reboot the system and wait for it to crash to the Blue Screen. Continue to use your system normally, and if you know what causes the crash, do that repeatedly. The objective here is to get the system to crash because Driver Verifier is stressing the drivers out. If it doesn't crash for you, then let it run for at least 36 hours of continuous operation (an estimate on my part).

If you can't get into Windows because it crashes too soon, try it in Safe Mode.
If you can't get into Safe Mode, try using System Restore from your installation DVD to set the system back to the previous restore point that you created.
Driver Verifier - Enable and Disable

Verifier puts extreme stress on the drivers, bad ones will cause BSOD. If we change all those drivers we hope for no more BSODs, If you get no BSODs, then its not a driver and we look to hardware. With verifier on your computer may be a little laggy, but actually..its just doing its work.

Information
Driver Verifier runs in the background, "testing" drivers for bugs. If it finds one, a Blue Screen of Death (BSOD) will result; the corresponding dump file will hopefully show the faulty driver.

Capt.Jack Sparrow said:

Driver Verifier monitors kernel-mode drivers and graphics drivers to detect illegal function calls or actions that might corrupt the system. It can subject the drivers to a variety of stresses and tests to find improper behavior.

Continue to use your system normally, and if you know what causes the crash, do that repeatedly. The objective here is to get the system to crash because Driver Verifier is stressing the drivers out. If it doesn't crash for you, then let it run for at least 36 hours of continuous operation.