This method becomes useless after the death of Windows. E.g. "Cannot load, missing or corrupt system32configsystem " message at boot. Even if you have another operating system installed, you cannot reload your backup to the registry. If you have a second WinXp installation, you should try mounting the corrupt reg archive (system, software, etc.....) and import your backup there. But this is risky. If you do not have SP2 installed (on XP), you should use ERUNT for XP and 2000. That's the best utility for that.
Note: Windows has a backup utility that can backup registry archive files. These can be found in "WindowsSystem32config"