Here are a few things to try. In the following commands, C: represents the hard disk and D: the CD/DVD drive, change them as necessary to suit your configuration.
1. Boot the XP CD and select the first repair option (R) which will take you to the Recovery Console. Now copy the missing/corrupt file from the CD to your hard disk by typing ‘copy D:\i386\ntoskrnl.exe C:\Windows\system32\ntoskrnl.exe’ (not the quotes but
the two spaces) and press Enter.
2. If you can only find ntoskrnl.ex_ on the CD then the file needs to be expanded by typing ‘expand D:\i386\ntoskrnl.ex_ C:\Windows\system32\ntoskrnl.exe’ (not the quotes but the two spaces) and press Enter.
If 1 and 2 above do not work, rename the file by typing ‘ren C:\Windows\system32\ntoskrnl.exe C:\Windows\system32\ntoskrnl.old’ (not the quotes but the two spaces) and press Enter. Then try again.
3. If it’s still not working, type chkdsk /r (note the space) and press Enter.