a) Insert the Windows XP CD and restart the computer (first set the bios setup with the cd rom as the first option on BOOT/options). Don't forget press any key when the windows requested 

b) In the first blue windows setup there is three options...press R=Repair option. Press the number that corresponds to the correct location for the installation of Windows you want to repair (Typically this will be #1) and enter the administrator password when requested

c ) type each command an press enter:
cd \windows\system32\config
ren software software.bad
copy \windows\repair\software

Take out the CD ROM and type exit