WinPE

Windows Preinstallation Environment (also known as Windows PE and WinPE) is a lightweight version of Windows used for the deployment of PCs, workstations, and servers, or troubleshooting an operating system while it is offline.

It is intended to replace MS-DOS boot disks and can be booted via USB flash drive, PXE, iPXE, CD-ROM, or hard disk.

Traditionally used by large corporations and OEMs (to preinstall Windows client operating systems on PCs during manufacturing), it is now widely available free of charge via Windows Automated Installation Kit (WAIK).

More about WinPE @Microsoft.

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How to Make Your Own Windows PE Rescue Disc (And Keep Your PC Safe)