Windows XP

Windows XP (codenamed Whistler) is an operating system developed by Microsoft, released to manufacturing on 24 August 2001 and general availability on 25 October 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 0.67% of desktop and laptop computers worldwide as of November 2021. It is also the last client version of Windows that supports computers without ACPI.

Even though extended support ended on 8 April 2014, a total of three out-of-band updates were serviced to the operating system fixing the following issues:
 * a high-risk Internet Explorer vulnerability; released in 2014
 * mitigations for the ETERNALBLUE SMB vulnerability, notably used by the WannaCry ransomware; released in 2017
 * mitigations for the BlueKeep RDP vulnerability; released in 2019

Windows XP lost its remaining support on 31 August 2019.

User interface
Windows XP recieved a major UI overhaul during development through the introduction of visual styles. 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 (2000, Me, and earlier), 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, the 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 Windows 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. It 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. The KN edition is the Korean counterpart to the N edition.
 * Reduced Media Edition: Identical to the aforementioned N editions. It was the initial branding given to the version before the European Commission had ordered Microsoft to change the name as it was deemed not appealing to consumers.

Myths and legends
No builds of Whistler ever used or even included the widespread startup and shutdown sounds. These originate from a Whistler theme from ThemeWorld, which includes an entire sound scheme of sounds from various sources such as Microsoft Plus! 98, although these are not widespread. 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 the Microsoft Entertainment Pack, while the shutdown sound came from BeOS albeit downsampled.

In reality, all builds up until 2485 use the same sounds as Windows 2000 and Me. Build 2481 introduced the sounds used in the final release (albeit in stereo and 44khz), but they were not used by default at this point.

Source code leak
On 23 September 2020, the source code for the RTM builds of Windows XP Service Pack 1 and Windows Server 2003 leaked on 4chan's /g/ board. As mentioned on the original post, the code had been circulating in private circles for several years at most. Due to the incompleteness of both source code repositories, primarily within the activation functionalities, it is fair to assume that the disclosure had originated from a Microsoft Partner who had access to the source code 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.