You can always try to just delete the files, leave the registry be. A notch more sophisticated would be to delete the files but the main entry point, and replace that entry point with a script named the (case insensitive) same. All the script should do is parsing any arguments to IE and if possible convert that to a parametrized call to the Firefox program. So if any app tries to launch IE it will in fact be tricked into launching Firefox -- though possibly not everything will work in the app that launches the browser.
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