Sounds as though the only realistic option is a complete reinstall from either a bootable Win 10 DVD or USB memory stick. I appreciate it might take some time but trying to fix the existing installation will probably take a lot more time and with no guarantee of success.Download the latest Media Creation Tool from MS. I believe it will still download Version 1803 not the latest screwed up Version 1809. You should not require a serial number as if Win 10 was already on the machine then it will be already registered on MS activation servers.
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