Windows Longhorn build 4001

Windows Longhorn build 4001 is the first Milestone 4 build of Windows Longhorn. This build was first found from screenshots that were shown by Dr. Nicholas Rush at YouArePwned's "Yet Another Longhorn Board", and by user bashar2000 on the BetaArchive forums. . It was eventually leaked to the aforementioned website on 15 January 2013.

While some features such as the Desktop Composition Engine and Start Page is mostly retained from build 3718, it notably contains a new setup procedure. This build also features the old M3 background, where it would be changed in build 4002.

OOBE
This build contains a new OOBE that replaced the Windows XP OOBE used in previous builds. The OOBE in this build only asks for account names to create (these will be Administrator accounts) and then finishes.

WIM installation
This build is among one of the first builds to use a new WIM-based setup routine, one of the first radical departures from the previous Windows NT codebase. The WIM-based setup routine works by having all of the setup files stored within a single WIM disk image,. When the installation media is booted directly, the WinPE (Windows Preinstallation Environment) loads into the graphical user interface itself. After it images the WIM file to the system partition, the system reboots to finish the setup by installing the hardware and running the OOBE. Microsoft stated that their goal with this routine was to allow installations of Windows to finish in as little as 15 minutes.

While the installation is basic (using the .NET interface) and lacks partitioning, it is capable of detecting hardware (including the CPU). As a result, it cannot be installed on processors older than a Pentium III, alongside a few virtual CPUs like the default emulated QEMU processor. The processor requirement would be removed in all subsequent pre-reset builds for unknown reasons.

Installation

 * Many drivers aren't installed by default during the hardware portion of setup and have to be installed manually.
 * As this build's setup cannot partition the drive, it is required to partition the drive before installation via the  command.
 * The setup doesn't ask for the username, organization information, and time zone, the default username of "Windows User" without an organization is used in place. Alongside, the time zone defaults to the Pacific Time Zone.

Taskbar
An interesting quirk with the New Taskbar application is that buttons appear to not be centered.

Desktop Composition Engine
Desktop Composition Engine requires either compatible hardware or VMware Workstation 6.5.x to work (just 6.5x compatibility won't work), otherwise it will result in a black screen that can only be exited by a hard reset.

Media Center
Due to the new WIM setup method, Media Center cannot be installed on this build.