Sometimes windows is unable to start because the registry gets corrupted and you get an message like

Windows XP could not start because the following file is missing or corrupt:\WINDOWS\SYSTEM32\CONFIG\SYSTEM


Or something similar .to fix this either you can reinstall windows OR recover the registry

To start up your machine without reinstalling, you need to execute some commands at the recovery console. To get to the recovery console, boot with the help of a bootable Windows XP CD, and press [R] when prompted to enter the recovery console. Select your Windows folder, and enter the administrator password. Once at the console prompt, the first thing to do is back up the existing file; you do so by executing the following commands at the prompt:

md tmp
copy c:\windows\system32\config\system
c:\windows\tmp\system.bak

Now you need to replace these files with the proper, non-corrupt files,which are in the repair folder in the Windows directory:

copy c:\windows\repair\system c:\windows\system32\config\system

Now you can boot into Windows XP,but, as this file was backed up when Windows was originally installed, any changes and settings after that are lost.If System Restore is enabled on your system, you can get back to the last restore point. To do this, boot into Safe Mode with administrator privileges, go to the drive
where Windows XP is installed, and look for the”System Volume Information” folder. Look for a snapshot folder for example, C:\System Volume Information\restore{C0B08955-BE89-4849-ABC0-715FCDE6CE4A}\RP3. Copy the _REGISTRY_MACHINE_SYSTEM file to the folder by typing in copy _REGISTRY_MACHINE_SYSTEM c:\windows\tmp\_REGISTRY_MACHINE_SYSTEM.Go to the recovery console again,using the Windows XP boot disk,and copy this file over the existing one by typing

copy c:\windows\tmp\_registry_machine_system c:\windows\system32\config\system

Restart Windows normally, start System Restore and restore your machine to any point you wish.