ntoskrnl.exe is a system file that contains the boot screen image, that is, the image that is displayed while booting in windows Xp.
This file may become corrupt, if you try to download additional boot screen images from web and try to replace them with the ones in your computer. This occurs because the ntoskrnl.exe file will have to be edited to change the boot screen image.
If you download a boot screen image from the web and replace your original ntoskrnl.exe then this file may get corrupted.
This is because this file while modification, might have been taken from a different windows version. For example, the modified boot screen might have been taken from windows Xp SP2. But if you apply the same ntoskrnl.exe file in your SP3 windows Xp computer, then the version might conflict and hence windows gives the error message.