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 for high-end users and Windows Me for home users and preceding Windows Vista for both consumer and media center editions. 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 0.38% of computers worldwide as of August 2022. Windows XP is the final client version of Windows which supports computers without ACPI and to have the ability to use boot disks to boot into setup.

Although 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 9 April 2019.

User interface
Windows XP received 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.

Windows Explorer received a major overhaul in its design and functionality with the introduction of the task pane which presents the user with a list of actions that a user can take depending on the selected item. Search Companions were also introduced to make searching more easier and there are four search characters available; Rover, Merlin, Earl and Courtney. They utilize the Microsoft Agent technolgy which is also used in the Office assistants. However, the classic search pane can still be restored by editing the registry. The filmstrip view has also been added along with a thumbnails viewing mode. Metadata of files can also be viewed from the properties page as well. Support for creating and opening compressed folders and Cabined files was also added along with the ability to burn CDs and DVD-RAM discs.

To improve boot times and program launch times, prefetching was added and it accomplishes this by caching files that are needed by an program to RAM during the program's launch, thus consolidating disk reads and reducing disk seeks.

The Compatibility modes were also added and are now activated out of the box. This feature allows users to run older programs by selecting an earlier version of Windows that the program previously ran on. This basically tricks the program into thinking that it is running on an older version of Windows.

Fast user switching was also added and allows users to login into the computer without having existing users to close out of their programs and log out.

A DRM technology called Windows Product Activation was added which requires the user to activate their copy of Windows XP as a means of enforcing the relevant provisions of the license agreement about using the product on multiple computers. Activation could be done over the internet or over the telephone. If the user does not activate Windows XP after 30 days of installation, the user will be unable to login to Windows unless it is activated. A unique ID is assigned to the computer after activation and if the user makes significant changes to the hardware, the activation is voided and the operating system must be re-activated.

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.

A new Start menu with two columns was introduced, which has the one column for pinned or recently opened apps and docs and the other one with shortcuts to certain places in the system. The user name and the new user picture are displayed at the top, while the buttons for logging off, undocking or shutting down the computer are located at the bottom. It is designed for use with large screens and the Luna theme, but it also can be used with the Windows Classic one. Additionally, the user can revert to the classic Start menu, where, apart from new shortcuts, icons and banner, almost nothing has been changed.

Applications and components

 * Internet Explorer 6 was introduced.
 * MSN Explorer was added for the first time to the Windows NT line after being ported from Windows Me.
 * Outlook Express 6.0 was introduced.
 * Windows Messenger was added.
 * Remote Assistance was added which allows a user to view or control a remote Windows computer over a network or the Internet to resolve issues.
 * Windows Picture and Fax Viewer was added, replacing Imaging.
 * The Language Bar was added.
 * Windows Media Player 8 was introduced.
 * Windows Movie Maker 1.1 was introduced.
 * Terminal Services Client was renamed Remote Desktop Connection.
 * The Fax console was added.
 * Windows Address Book was added.
 * Windows NetMeeting 3.01 was introduced.
 * WordPad now has full Unicode support.
 * Desktop Cleaning Wizard was added.
 * WinPE was introduced.
 * DirectX 8.1 was introduced.
 * Windows Error Reporting was introduced.
 * The ability to easily roll back a device driver to an old version was added.
 * Wireless Zero Configuration was introduced.
 * Password reset disks can be created to reset the user account password.

Deprecations

 * Windows XP drops the ability to upgrade from both Windows NT 3.51 and Windows 95, requiring at least Windows 98 or Windows NT 4.0. SP2 (or later) also drops the ability to upgrade from Windows 98 due to a bug in the upgrade program, now requiring Windows Me.
 * Links to Phone Dialer and Windows NetMeeting have been removed. They can still be accessed and used.
 * The AppleTalk protocol was completely removed.
 * WebTV for Windows was removed.
 * The QuickEdit and Insert modes in the Command Prompt have been disabled by default.
 * The Small icons view was removed from Windows Explorer.
 * Deluxe CD Player and DVD Player have both been removed and replaced by Windows Media Player.
 * Imaging for Windows was removed in favor of Windows Picture and Fax Viewer.
 * The Directory icon was removed from My Network Places.
 * The NetDDE and NetBEUI protocols are no longer included by default.
 * The DLC network protocol is no longer included by default.
 * The Microsoft POSIX and OS/2 subsystems were removed.
 * The DriveSpace utility was removed in favor of native NTFS compression.
 * The progress bar on the boot screen is no longer determinate.
 * The Line Up Icons command on the desktop was replaced by the Align to Grid command.
 * The Customize This Folder wizard was removed.

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 2002-2007. 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.

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 Windows 98 Plus!, although these are not widespread. The fake startup sound was made for the previous one, using Windows 98 Plus!'s World Traveler, Architecture shutdown sound, and PhotoDisc startup sound, and the final version 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 slowed down, while the shutdown sound came from BeOS albeit down sampled.

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 44.1kHz), but they were not used by default at this point.

System requirements
According to Microsoft, Windows XP requires a Pentium processor, at least 64 MB of RAM, 1.5 GB of hard drive space, and a VGA or better display adapter. Windows XP drops full support for processors without the CPUID instruction, like the 486. Additionally, the HAL for the SGI Visual Workstation 320 and 540 is no longer included in Windows XP. Windows XP updates the default VGA driver to take advantage of VESA BIOS extensions, allowing true color display and resolutions in the default VGA driver, although this requires a graphics card that supports SVGA.

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.