Microsoft.com accounts made their debut in W8.

But even Vista already had some nasty features like IE Smartscreen which to this day is on by default and which sends every website URL you visit to Microsoft. Vista was also the first Windows version to include telemetry throughout the OS. However, in Vista and W7 you could still disable telemetry on normal editions of Windows.

From a privacy standpoint, the last good-by-default OS was XP. The only bigger issues iirc were the Media Player which downloaded album art and DRM licenses and Active Desktop which Microsoft tried to use to advertise to you. Oh, and (edit): Windows license activation was online for the first time and in some cases you had to reactivate after changing hardware.