In Control Panel>System towards the bottom of the page is your Activation ststus. If it isnt a genuine MS product, it will have fail activation and you will need to go to Microsoft's site to buy a licence key. If you have to do that, it makes sense to buy the most recent vesion, Windows 10. Buying from anywhere else runs the risk of getting caught out with a dud system.
If the system shows it has been successfully activated, you're clean.