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If you make changes to your system that corrupts the registry and you cannot start windows, boot to the command prompt and type "scanreg" without the quotes.
This will walk you through scanning the registry, after scanning the registry, if it finds a problem, it will find a copy of the registry that was in working order.
You can find more details within these MSKB articles--
Description of the Windows Registry Checker Tool (Scanreg.exe)-(Q183887)
Command-Line Switches for the Registry Checker Tool-(Q184023)