a) Insert your Windows XP CD (first set the bios setup for to boot from the cd rom as first option) and restart the computer (don't forget to press any key when is requested) 

b) In the first blue windows there is three options...press R=Repair option. Then press the number that corresponds to the correct location for the installation of Windows for to repair (Typically this will be #1)

Enter the administrator pasword when requested (If not any press enter) 

c) type:

cd \windows\system32\config  (enter)
ren system system.bad               (enter)
copy \windows\repair\system   (enter)

Take out the CD ROM and type exit