if you get this messagge at boot-up, means that your file NTOSKRNL have some troubles.

• first you need the CD of your Windows XP, reboot the machine (be sure your BIOS can start-up from cd).
• run the setup like a new windows installation until the setup ask you to press F8 for accept the Eula .
• Next the setup will search if there are another OS installed.
• When setup fin your existing OS, press R (Restore) for gain access to restore console.
• select the windows (usually option 1) give the administrator password if you have one.
• Now you have a prompt, be sure you are inside cdrom path, the write:

CD i386
expand ntkrnlmp.ex_ C:\windows\system32\ntoskrnl.exe
(or whatever you have installed windows, if not in c:\windows)

• when completed (you see another time the prompt), extract the cd and type:

exit

• now the machine reboot correctly.