a) Insert your windows xp cd and restart the machine (first set de bios with cd rom in top of the BOOT options)...and press any key when the chart ask it 

b) At the first blue windows press R=Repair option and press the number that corresponds to the correct location for the installation of windows for repair (Typically will be #1)

c) For to go to the CD rom drive: type  

CD i386 (enter)

expand ntkrnlmp.ex_ C:\Windows\System32\ntoskrnl.exe  (enter)

If your Windows is installed in other drive change the letter C for the other letter

Take out the Windows CD and type exit