Ntoskrnl.exe Missing or Corrupt XP
Try the following method to fix this ntoskrnl.exe problem.Hope this will fix your problem
1.Start the computer by using your Windows XP CD-ROM. Press any key to boot from the CD.
2.After the setup files are finished loading press R to repair using Recovery Console.
3.When you are in the recovery console, select the installation to log on to (usually number 1), and then press ENTER.
4.Login to the Administrator account by typing the password for this account, and then press ENTER.
5.At the recovery console command prompt, type the following command, and then press ENTER:
For Uni-Processor systems:
expand :\i386\ntoskrnl.ex_ :\Windows\system32\ntoskrnl.exe For Multi-Processor systems:
expand :\i386\ntkrnlmp.ex_ :\Windows\system32\ntoskrnl.exe Note In these two commands, the placeholder represents the drive letter of your CD drive, and the placeholder represents the drive letter of the hard disk on which windows is installed.
6.If you receive a prompt to overwrite the file, press Y.
7.Type exit, and press ENTER at the command prompt.
This method from microsoft support .If this does not work check the following help link