What else do you need to know?
You might be wondering what happens if you upgrade your current PC to Windows 10 and then decide to build a new machine? Given that Windows 10 will be tied to your current machine, we don't imagine you'll be able to transfer the activation to another system.
How does that translate to smaller hardware changes? That remains a bit of a grey area, but we've changed the hard disk in one of our Windows 10 PCs and we've been able to carry out another clean install and activate without a problem. We suspect that bigger changes - a new motherboard, for example - may scupper the activation but we've yet to confirm that this is the case.
What if you're running Windows XP or Windows Vista? Sadly, you're out of luck - the free upgrade offer is only applicable to users of Windows of Windows 7 and Windows 8.
What if you've bought a retail copy of Windows 10? You can go straight to Step 3 and clean install using the product key that came with your software.
And then there's the big question: should you even bother upgrading to Windows 10? It is, in our estimation, shaping up to be the best Windows to date, however it's also unique in that this is the first Windows to be delivered as a service. What this means is that Windows is changing from a product that sees periodic major releases to one that's frequently being updated. As such, Windows 10 is never entirely finished, and at launch there are still bugs being squashed - there's already been a day-one patch weighing in at over 1GB.
Waiting for the quirks to be ironed out isn't the worst idea, and the free upgrade offer is currently scheduled to run until July 29, 2016, so there's really no hurry. And who knows, other, less-convoluted methods of clean installing may become available in that time.