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

Three days ago, a friend asked me to recover his laptop (winXP) from the following error which always appeared during booting:

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

The solution is straightforward from Microsoft website (http://support.microsoft.com/kb/307545). In brief:
The first step is to gain access to windows OS by replacing the broken registry file with the default registry file (windows installation CD is required). The second step is to copy the latest system restore registry file (hopefully your system restore is enabled) to replace the default registry file.

To be more detail, those steps are done as follow:

1. boot using the WinXP installation CD, select repair


2. after entering the command prompt, manually copy (overwrite) the registry file (SYSTEM, SOFTWARE, SAM, SECURITY, DEFAULT) from C:\WINDOWS\REPAIR\ to C:\WINDOWS\SYSTEM32\CONFIG\ (you might want to backup the original files)


3. restart to save mode.


4. gain access to System Volume Information folder (http://support.microsoft.com/kb/309531/), inside the folder, find sub folder with name _restore {GUID}\RPx\snapshot. For example: C:\System Volume Information\_restore{D86480E3-73EF-47BC-A0EB-A81BE6EE3ED8}\RP1\Snapshot


5. copy the following files from the Snapshot folder to any other folders: _REGISTRY_USER_.DEFAULT, _REGISTRY_MACHINE_SECURITY, _REGISTRY_MACHINE_SOFTWARE, _REGISTRY_MACHINE_SYSTEM, _REGISTRY_MACHINE_SAM. Rename each of the files to DEFAULT, SECURITY, SOFTWARE, SYSTEM, SAM, respectively.


6. boot using WinXP installation CD, select repair


7. after entering the command prompt, manually copy (overwrite) the file from step (5) to C:\WINDOWS\SYSTEM32\CONFIG\


8. restart.


9. finish, or according to the microsoft website, you should try to restore the setting to previous restore point (Start, All Programs, Accessories, System Tools, System Restore, Restore to a previous restore point).

That's all :)