I purchased the PC I'm currently using several years ago. Alongside with it, I purchased Windows 7.
A while back Microsoft offered a free upgrade to Windows 10 to all Windows 7 (and 8) users. (They really, and somewhat obnoxiously, pushed this upgrade onto people, but whatever; that's not the subject of this post.) I took the upgrade, because why not.
The offer of the free upgrade was a limited time thing, though. I think it was a year or something like that. As far as I know, they are not offering it anymore, meaning that if you today want to upgrade from Windows 7 or 8 to 10, you'll need to purchase the latter.
This got me thinking: I don't actually have a physical copy of Windows 10. It was an upgrade over the installed Windows 7 done through the internet. I only have the physical disk for Windows 7. So what happens if for whatever reason I need to reinstall Windows from scratch? (For instance, the most likely scenario for this is if my hard drive completely breaks.) Would I need to purchase Windows 10, if I want to keep using it? Or would I be stuck with Windows 7?
However, I did some research on this subject and it appears that if you own a valid product key for Windows 7 (which comes with the physical disk), you can download and install Windows 10 from Microsoft and register it using that key. That's quite nice.
However, apparently, it goes even farther than that. You actually don't need the Windows 7 product key either. As long as you have had a legal version of Windows 10 installed in your PC, you can re-download and re-install it on that same PC, and it will detect that it had been legally installed before, and will register itself again. Therefore even if you lose your original installation disk, with the product key, you will still be able to re-install Windows into the same PC.
I don't know exactly how it recognizes that it's the same PC, but I'm supposing it's using a cpu-id, and perhaps other uniquely identifying hardware info, that it has sent to Microsoft when you registered Windows.
This, of course, raises contradictory feelings.
On one hand it's really nice to have this kind of safety. If for example your hard drive fries, you don't have to worry about having to purchase Windows again, if you don't have the original disk. Microsoft could have easily gone the greedy route and said "oh, your computer fried? Well, sucks being you. You'll have to pay us again for Windows." But instead, they are being really fair, and if you have purchased Windows once, you don't have to do it again (at least if you are installing it on the same PC.)
On the other hand, Microsoft has uniquely identifying information about your PC. Microsoft can, if they so choose, see pretty much everything you do with your PC and identify exactly which PC it's coming from. Any person with a sense of privacy ought to get shivers going down their spine.
One has to simply trust that a corporation like Microsoft is not going to abuse this kind of information. Given Microsoft's less-than-stellar reputation, they haven't exactly earned this kind of trust, at least not fully.
So it really is a dilemma. On one hand they are being incredibly fair by offering this kind of service where you don't need to purchase Windows again due to an accident, which is very nice. On the other hand, they have uniquely identifying information about your PC, which is a bit scary of a thought.