NTOSKRNL.EXE is missing or corrupt

1. Insert the Microsoft Windows XP CD. Note: If you have a recovery CD or a restore CD and not a Microsoft Windows XP CD it is likely the below steps will not resolve your issue.
2. Reboot the computer, as the computer is starting you should see a message to press any key to boot from the CD. When you see this message press any key.
3. In the Microsoft Windows XP setup menu press the R key to enter the recovery console.
4. Select the operating system you wish to fix, and then enter the administrator password.
5. Type expand d:\i386\ntoskrnl.ex_ c:\windows\system32
6. You will then be prompted if you wish to overwrite the file type Y and press enter to overwrite the file.
7. Type exit to reboot the computer.

Source : http://www.computerhope.com/issues/ch000646.htm