The registry, NTFS, the security model, and the enterprise tooling behind Microsoft's OS.
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.
Pooling multiple physical drives into a resilient virtual disk - Windows' built-in, software-defined answer to hardware RAID, set up step by step.
Released July 29, 2015 as a free upgrade for Windows 7/8.1 users, Windows 10 reached over 75 million devices within a month on a bet on rapid adoption.
Released to manufacturing July 22, 2009 and to the public that October, Windows 7 refined Vista's foundations into one of Microsoft's most successful releases.
How a modern Windows machine goes from firmware to a running kernel, and where each stage's configuration actually lives.
How discretionary ACLs, mandatory integrity levels, and UAC's token-splitting combine to form Windows' layered access control model.