I don't run Windows personally, but I know that I've seen articles on Metalink describing how to deinstall products on Windows by hacking the registry.
Given the uncertain history of your workstation, it may be faster for you to reinstall Windows and then install the software you need to do your job. You won't learn any useful skills by hacking the registry to remove Oracle software, and installing the software on a "clean" system may be more beneficial in both the short and long runs.
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