I'd reinstall for the main reason you give: registry entries. Also, you'll end up with a lot of broken links otherwise, including all of your application icons not finding their respective application executables. If you know that an application doesn't create registry entries and maybe it doesn't have a shortcut icon for launch, you might be able to copy it without ill effects - this is usually only possible for those applications that you just extract from a zip into a folder and don't "install" anything when run.

C makes it easy to shoot yourself in the foot. C++ makes it harder, but when you do, you blow your whole leg off.

 -- Bjarne Stroustrup

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