Lots of people use an android or an iPhone as their main computer nowadays. If you're targeting keyboard/mouse style input, then Windows is probably close to as popular as ever. But if you're targeting people using some kind of device to access your service in exchange for money, Windows is wasting your time.
Any PC from the past decade is still mostly serviceable.
I'm finally upgrading from an Intel Sandy Bridge processor after nine years, and I still don't need to - it's cranking along pretty well as a dev and gaming machine still.
