Boot your system with the Windows XP Install CD, let the system boot into the
Setup. Once in the Setup, choose to run the Recovery Console.
You will now be presented with a screen similar to good old DOS.
First, we will recover the System Hive.
Now, type in the following commands with pressing Enter after each line.
copy C:\windows\system32\config\system C:\windows\tmp\system.bak
copy C:\windows\repair\system C:\windows\system32\config\system
Be very careful when you are typing in these commands, one wrong move, and you have, broken Windows XP.
Also, change the C in the commands to whatever your Windows XP drive letter is.
Attempt to boot the system, if it doesn't boot, get back into the Recovery Console and do the following commands.
copy C:\windows\system32\config\software C:\windows\tmp\software.bak
copy C:\windows\repair\software C:\windows\system32\config\software