My first PC was an Acorn Archimedes A3010 which ran on RISC OS/2. When I switched to an IBM-compatible PC I started with Windows 95 and worked my way up to Windows 10 (with a bit of Linux Mint on the side).
i'm on windows 10 too but not through choice. my windows 7 was OEM and i built a new PC so had to buy windows again. blergh. it's ok but my big complaint is they dropped support for lots of hardware such as my super posh audio interface! every time there's an update it breaks my sounds! this is STILL a problem if i say for example get any work in my capacity as a freelance musician. i could rant forever about this but lets not do that.
as for first? techinally my first PC had windows 2000/XP on it but the first PC i used in the household was a DOS/3.1 one. because i'm old too.