This file contains valuable registry settings for your computer, to prevent it do this (instructions):
Refer to MS KB article Q307545, to see how to get a Restore point to get the registry to the last known usable state--unless you enjoy or want to re-install all of your applications (to reset registry entries. All data is safe.
How to Recover from a Corrupted Registry that Prevents Windows XP from Starting (Q307545)
The information in this article applies to:
Microsoft Windows XP Home Edition
Microsoft Windows XP Professional
SUMMARY
This article describes how to recover a Windows XP system that does not start because of corruption in
the registry. This procedure does not guarantee full recovery of the system to a previous state; however, you should be able to recover data when you use this procedure.
You can repair a corrupted registry in Windows XP. Corrupted registry files can cause a variety of different error messages. Please refer to the Knowledge Base for articles regarding error messages related to registry issues.
This article assumes that normal recovery methods have failed and access to the system is not available except by using Recovery Console. If an Automatic System Recovery (ASR) backup exists, it is the preferred method for recovery; it is recommended that you use the ASR backup before you try the procedure described in this article.
NOTE : Make sure to replace all 5 of the registry hives. If you only replace a single hive or two, this can cause potential issues, since software and hardware may have settings in multiple locations in the registry.
MORE INFORMATION
When you try to start or restart your Windows XP-based computer, you may receive one of the following
error messages:
Windows XP could not start because the following file is missing or corrupt: \WINDOWS\SYSTEM32\CONFIG\SYSTEM
Windows XP could not start because the following file is missing or corrupt: \WINDOWS\SYSTEM32\CONFIG\SOFTWARE
Stop: c0000218 {Registry File Failure} The registry cannot load the hive (file): \SystemRoot\System32\Config\SOFTWARE or its log or alternate
The procedure described in this article uses Recovery Console, System Restore, and lists all the required steps in specific order to ensure that the process completes fully. After you complete this procedure, the system should return to a state very close to the system before the problem occurred. If you have ever run NTBackup and completed a system state backup, you do not have to follow the procedures in parts two and three; you can skip to part four.
Part One
In part one, you boot to the Recovery Console, create a temporary folder, back up the existing registry files to a new location, delete the registry files at their existing location, and then copy the registry files from the repair folder to the System32\Config folder. When you are finished with this procedure, a registry is created that you can use to boot back into Windows XP. This registry was created and saved during the initial setup of Windows XP, so any changes and settings that took place after Setup completes are lost.
To complete part one, follow these steps:
Boot to the Recovery Console.
At the Recovery Console command prompt, type the following lines, pressing ENTER after you type each line:
md tmp
copy c:\windows\system32\config\system c:\windows\tmp\system.bak
copy c:\windows\system32\config\software c:\windows\tmp\software.bak
copy c:\windows\system32\config\sam c:\windows\tmp\sam.bak
copy c:\windows\system32\config\security c:\windows\tmp\security.bak
copy c:\windows\system32\config\default c:\windows\tmp\default.bak
delete c:\windows\system32\config\system
delete c:\windows\system32\config\software
delete c:\windows\system32\config\sam
delete c:\windows\system32\config\security
delete c:\windows\system32\config\default
copy c:\windows\repair\system c:\windows\system32\config\system
copy c:\windows\repair\software c:\windows\system32\config\software
copy c:\windows\repair\sam c:\windows\system32\config\sam
copy c:\windows\repair\security c:\windows\system32\config\security
copy c:\windows\repair\default c:\windows\system32\config\default
NOTE : This procedure assumes that Windows XP is installed to the C:\Windows folder. Make sure to
change C:\Windows to the appropriate windows_folder if it is a different location.