Windows 8.1

Windows 8.1 is an operating system by Microsoft. It is a free upgrade for existing Windows 8 users. It replaced Windows 8 and was succeeded by Windows 10 on 29 July 2015. This is the first version of 64-bit Windows to drop support for processors without the CMPXCHG16b, PrefetchW and LAHF/SAHF instructions, as running on these processors will result in a bugcheck with error code 0x5D.

Although the operating system was marketed by Microsoft as an update for Windows 8, Microsoft's traditional support policy treats Windows 8.1 as a service pack.

It is the last release of Windows to have support for Windows Media Center as Microsoft discontinued support for the application with the release of Windows 10. It is also the last release of Windows to have the ability to create MS-DOS 8 startup disks, and thus to include MS-DOS in any form.

RTM

 * The Start button was brought back (although it still leads to the Metro start screen).
 * Computer is renamed to This PC.
 * Clicking the File Explorer icon in the taskbar takes you to This PC instead of Libraries, unlike in Windows 7 and 8. Additionally, Libraries is disabled by default, you can re-enable it by right-click on the navigation pane and choose "Show libraries" or "Show all folders".
 * User folders (Desktop, Documents, Downloads, Music, Pictures, Videos) added to This PC.
 * The Windows Experience Index has been removed. However, the command-line tool to determine the rating values (WinSAT) is still included.
 * New set of default wallpapers.
 * New customization options for the Start screen.
 * New Metro applications introduced, with many existing ones updated.
 * Redesigned PC Settings.

Update 1 (on non-touch devices)

 * When you hover over the top of a Metro app, Windows shows a title bar with minimize and close buttons.
 * Right-clicking on the Start screen opens up a menu instead of the touch-oriented controls at the bottom.
 * The "Go to the desktop instead of Start when I sign in" feature in Taskbar and Navigation Properties first introduced in the original release is now enabled by default.
 * You can pin Metro apps to the taskbar.
 * A power button has been added next to the search button on the start screen.
 * Added dividers between apps in the "All Apps" view in Start.
 * SkyDrive is renamed to OneDrive (you can also get this in Windows 8 and Windows 8.1 RTM (without Update 1) by updating the SkyDrive app via the Store).
 * The default media player and photo viewer has been changed to Windows Media Player and Windows Photo Viewer respectively.

Update 1 pre-release builds
These builds aren't have a timebomb