The file ntoskrnl.exe is a core Windows operating system file. If it's corrupt or missing your PC will not be able to startup. The solution is to Insert your Windows installation disc, restart your PC, and select "Repair your computer". If you don't have an installation disc, you might be able to copy the file from another computer.
The ntoskrnl.exe resides in the \Windows\System32\ folder.
The size of the ntoskrnl.exe is different with different Windows versions and service pack levels. Make sure the PC that you're copying the file from is the same Windows version and service pack level.
If you try copying the file from a live system you'll receive an "access denied" message. However, there are several methods you can use to access the file.
1. Boot with a Linux live CD like Ubuntu. It's also possible to make a Windows live CD.
2. Boot with Ultimate Boot CD for Windows which is a free bootable recovery CD that contains software used for repairing, restoring, or diagnosing almost any computer problem.
3. Take the HD out of the system and set it up as a secondary drive in another system.
Note: To boot from your CD, you need to set the CD as the first boot device in your system's BIOS. To do this, when your PC first starts, you'll see a message like "Press DEL to enter setup" or something similar. Immediately press the indicated key and wait for the BIOS setup screen to appear. On the BIOS setup screen select the Boot Device or Boot Order setting. Set that to CD/DVD. Save your changes when you Exit the BIOS setup.
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