Re: If I upgrade to windows 10 and I don't like it
"I still have my Genuine XP disc and the license key that goes with it. That means that I can install XP if I wish - and have hardware that can run it."
Or at least while MS keep the activation servers running...
The more interesting debate (and the reason I haven't upgraded) is the unclear scenario of when you try and transfer your "free" Windows 10 installation to a new PC. The best answer I can find is that MS will reject the authentication but a friendly agent would be able to activate your new installation over the phone.
If I buy Win 10 (likely case), I'll end up with a product key I can use how I like, similar to the 95, 98, XP and 7 keys I still have.