Still running Windows 7 or 8? You're eligible to upgrade free to Windows 10!
Microsoft has had the option of Windows 7 and 8 to upgrade to Windows 10 free of charge and in my opinion, 10 is far superior to the previous Windows systems that Microsoft has made in the past.
So you may ask, how does it work? You can either click on the link above or go straight to this one where they'll tell you all about it. Very simple really.
Avoid at all costs buying "cheap" Windows 10 keys because chances are they are pirated or stolen so they wouldn't work for too long.