Windows XP

Windows XP (codenamed Whistler) is an operating system developed by Microsoft, released to manufacturing on August 24, 2001 and general availability on October 25, 2001. It is the sixth operating system in the Windows NT operating system line, succeeding Windows 2000 and preceding Windows Vista. It also succeeded Windows Me after the end of the Windows 9x kernel. It is one of Microsoft's longest-lasting operating systems, with almost 13 years of support (both mainstream and extended) and still runs on 1.29% of desktop and laptop computers worldwide as of January 2020.

User interface
Windows XP got a major UI overhaul during development, with the introduction of Themes. Users could change the way windows and buttons looked with the new visual style, Luna. It had three color schemes, which were based on blue, green, and silver. Users could still select the Windows Classic theme from previous versions of Windows (98, 2000, Me, ...), and customize the preset Windows Classic color schemes.

The introduction of ClearType, an anti-aliasing font function allowed easier text reading on desktop computers with LCD-TFT monitors and laptops but also affected CRT monitors.

The taskbar is locked by default for new user accounts, to prevent accidental resizing or moving of the taskbar and/or the toolbars. Multiple instances of an application are grouped automatically and inactive tray area icons are hidden automatically, to prevent cluttering of the taskbar.

Editions

 * Starter Edition: This version was intended for emerging markets, and was introduced in 2004, meant for low-cost PCs. It cannot launch more than 3 programs at a time and won't recognize more than 512MB of RAM.
 * Home Edition: This edition is for home and personal users who don't need to use corporate network, security, and management features (such as joining domain networks, file encryption, Remote Desktop Connection, group policies, etc.) This version can address up to 4 GB of RAM, and it only supports a single physical processor (though multiple cores and threads are supported).
 * Professional: This is the mainstream high-end version, sold along with the Home Edition. It supports up to two physical processors.
 * 64-Bit Edition: This was a special edition for Itanium-based computers. There were two releases, first of which was based on Windows XP and the second of which was based on Windows Server 2003.
 * Professional x64 Edition: This was the version for x86-64 processors based on Windows Server 2003.
 * Media Center Edition: This version is especially for Media Center-based PCs, and is not sold normally. System requirements are significantly higher than in other editions. This version includes Media Center and new themes and Microsoft Plus! features such as Royale.
 * Embedded: This version is intended for the embedded sector.
 * Tablet PC Edition: This version was included in tablet computers from 2002-2006. This is the tablet-optimized version of Windows XP.
 * N, K and KN editions: The N version was made for the European markets and did not include Windows Media Player; the K version was made for the South Korean markets and included two shortcuts to Korean websites offering media player and messenger software by default, after a South Korean Fair Trade Commission ruling.

Startup and shutdown sound
No builds of Whistler ever used or even included the widespread startup and shutdown sound: In fact, all builds up until 2485 uses the same sounds as Windows 2000 and Me. Build 2481 introduced the sounds used in the final release, but it wasn't used as default at the point.
 * The fake startup sound was made using Windows 98's shutdown sound reversed, Windows 2000 build 1983.1 to build 2000.3's startup sound reversed, and the Next Level sound from a game titled Spring Weekend included in Microsoft Entertainment Pack.
 * The widespread shutdown sound came from BeOS.

Source code leak
On September 23, 2020, the source code for Windows XP Service Pack 1 and Windows Server 2003 RTM leaked on 4chan's /g/ board. As mentioned on the original post, this had been circulating in private circles for several years at most. Due to the incompleteness of both source codes, primarily on the activation side of things, it is fair to assume that this had originated from a Microsoft Partner who had the access rather than Microsoft themselves. The ability to glance into the groundwork of this operating system has led to some discoveries, an example being the checks for enabling the DirectUI-based Start Page from  being usable in build 2410.

Quirks
Windows XP is perhaps notable for being able to be installed on a failing hard drive. Older and newer OSes such as Windows 3.1 and Windows Vista do not work (with Vista and later telling you that the drive is failing). XP does not bring up any mention of the failing drive at any point during the installation and installs normally (Your computer's BIOS may tell you if it has that capability).