You could always reinstall the OS using the installation disk. However, this will wipe your computer completely clean. There are tools out there which you can use to repair your registry, but it's not guarenteed.
Make note that before you go toying around with the registry, you should ALWAYS back it up. You can do so by exporting the entire registry to the C: drive. This way, you can restore it to the previous point, even if you can only get into the system via Safe Mode.
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I swear, troubleshooting is a science: the best discoveries are always on accident...