You realize that it may, possibly, be a pirated version of Windows. Do you possess the license?
Even if not pirated before, it may be now!
"IF" the previous owner gave you the license and
the original installation media (disk, flash drive, ISO file - if applicable) AND
he or she deleted all
copies and installations of Windows from all his devices, AND
it was a full "Retail" license, then and only then is this transaction "legal".
But if that is an OEM license (as most are) and it was used to install to a computer (or came preinstalled on a factory made computer), then that OEM license is inextricably tied
to the "O"riginal "E"quipment - that is, the original computer.
Note when it comes to software licenses, it is the motherboard that is considered to be the computer (not drive). This is why when upgrading a computer, you can replace the drives, graphics card, CPU, RAM, and even the case and don't need to worry about the Windows license. But as soon as you upgrade the motherboard, that constitutes a new computer and may require the purchase of a new license.
The only time a OEM license can legally be transferred to a new owner is when the original computer is transferred with it - and
any and all backup copies made by the original owner are either deleted or go with the computer to the new owner.
So if any of those scenarios apply to you, do you need to worry that Microsoft is going to send the FBI or Interpol/Europol after you? No. They are not going to waste their time on a single individual. But they could!
What is very possible however, is Microsoft could invalid your license and render it inoperable.
So best to make sure you are legal here with a genuine license and copy. And for sure, if it was pirated before, then malware becomes (or should become) a major concern.