XP was IMO the inflection point, where Windows started to get worse faster than it got better. And close to my heart, Windows XP was the first version of Windows that was too big to fit on my trusty Compaq Presario 1210 and its whopping 1.4GB hard drive.
If Microsoft had "stopped" at Windows 2000 (like it has ostensibly "stopped" at Windows 10), and had focused on refining that piece of near-perfection, I'd probably still be a Windows user today, and happily so. My dissatisfaction with Windows - starting with XP/2003, but especially every version after - was the big reason why I looked into alternatives like Linux.