Probably bought a used computer with a used copy of Win10 installed.
Disable Remote Access.
https://www.lifewire.com/disable-windows-remote-desktop-153337Make sure Windows Firewall is running and properly configured. And make sure all "Remote Access" objects are Blocked unless you actually need to run them.
http://www.thewindowsclub.com/how-to-configure-windows-7-firewallScan the system to detect and repair/remove all malware, spyware, viruses, rootkits, and other yucky things.
http://www.pcworld.com/article/243818/security/how-to-remove-malware-from-your-windows-pc.htmlYour other option, of course, is a full (fresh, clean) Windows 10 install. But do not start this process until you have backed up all your important data (on some other drive) and you've written down your Windows Product Key. Having clean copies of all necessary drivers/software nearby also helps, especially those which are required to get your computer working well enough to boot Windows and access the internet.
http://www.techadvisor.co.uk/how-to/windows/how-find-windows-10-product-key-3632749/https://www.howtogeek.com/244678/you-dont-need-a-product-key-to-install-and-use-windows-10/You can reset your BIOS to factory defaults. You can even download and install (flash) a clean version from your motherboard page. But this shouldn't be necessary and (if you don't know what you're doing) could actually cause more problems by breaking things that already work. Write down all BIOS settings you don't understand (or can't easily figure out) before changing or resetting anything.
The chances of your BIOS being "infected" or "compromised" with some kind of malware or backdoor are virtually insignificant, but you may choose to overwrite it with a clean version anyhow "just to be sure".
There's other threads online which describe your problem, but they're not very informative.
https://www.reddit.com/r/Windows10/comments/6kyzcn/someone_registered_my_w10_keys_on_his_microsoft/And your final option is to contact Microsoft Support. You might be able to convince them to issue you a new Windows Product Key. Or you may be told there's nothing they can do until you buy a new one.
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