Mike,
Look in your Mach3 directory, sort the display by file type, and scroll all the way to the bottom. You should see just a few files of type "XML Document" (I only have 3). You'll probably have Mach3Mill.XML, LazyCAM.XML, and maybe a customized XML - that's the one you really need. If you don't have a customized XML, then Mach3Mill.XML is the one you care about. Just copy it to another directory, a USB drive, or wherever is convenient. After re-installing Mach3, copy it to the new installation directory.
There's no need to muck with the registry at all.
Regards,
Ray L.