Insert and boot from your WindowsXP CD.
At the first R=Repair option, press the R key
Press the number that corresponds to the correct location for the installation of Windows you want to repair.
Typically this will be #1
Change to the drive that has the CD ROM.
expand ntkrnlmp.ex_ C:\Windows\System32\ntoskrnl.exe
If WindowsXP is installed in a different location, just make the necessary change to C:\Windows
Take out the CD ROM and type exit