Windows Longhorn build 4001

Windows "Longhorn" build 4001 (with a build tag of build 4001.main.021204-1515) 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 BetaArchive on 2013-01-15.

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.

New features

 * This build contains a new OOBE that replaced the Windows XP OOBE used in previous builds. The OOBE in this build only asks for an account name 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.

Bugs and quirks

 * 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 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 Pacific Time Zone.