\WINDOWS\SYSTEM32\CONFIG\SYSTEM or \WINDOWS\SYSTEM32\CONFIG\SOFTWARE
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
Enter in the administrator password when requested
Depending on which section was corrupted:
ren software software.bad or ren system system.bad
Depending on which section was corrupted
Take out the CD ROM and type exit