The registry, NTFS, the security model, and the enterprise tooling behind Microsoft's OS.
First previewed as Monad in 2003, renamed Windows PowerShell in April 2006, and released that November — replacing cmd.exe scripting with an object-oriented shell.
Announced at Build 2019 and pushed to GitHub May 3, 2019, Windows Terminal brought tabs and modern rendering to the command line, fully open source.
UAC was a genuine security improvement, but its constant, poorly-tuned confirmation prompts made it the most complained-about feature of Vista's launch.
Replacing the 17-year-old Start menu with a touch-first screen alienated Windows 8's mostly desktop-and-mouse users, and Microsoft reversed course fast.
Why Microsoft's April 8, 2014 cutoff for Windows XP updates became one of the most consequential OS end-of-life dates, given how many machines still ran it.
Released to retail October 25, 2001, Windows XP was the first consumer edition built on the NT kernel, ending the split between the 9x and NT lines for good.
Why Windows services stopped interacting with the desktop starting with Vista, and what changed in the session architecture to cause that.
The coordination mechanism that lets Windows back up files that are open and being written to, without the inconsistency a live copy would risk.
Windows reports it isn't activated, or shows a specific error code after a hardware change or reinstall — read the code and apply the right fix.
Windows blue-screens with INACCESSIBLE_BOOT_DEVICE before the desktop loads — diagnose whether it's a driver, disk, or boot config problem from Recovery.