Since you're on Windows 10, you'll need a copy of it, and an activation key. The free upgrade is only offered to people on Windows 7 and 8.
However, if you do get Windows 10, slap it onto a USB/CD, get your activation key ready (Can be purchased through Microsoft), and just run it. The setup with set everything you need for it.
Note: Your PC might slow down significantly, however. The requirements for Windows XP and 10 vary greatly.