Microsoftは8月27日(現地時間)、Windows 11 Version 25H2/24H2用のプレビューアップデートKB5120998を公開しました。オプション扱いの更新プログラム(プレビュー版)で、多数の不具合が修正されています。Windows Updateの「更新プログラムのチェック」を利用するか、Microsoft Update Catalogからファイルをダウンロードすることで、手動でインストールすることができます。August 27, 2026—KB5120998 (OS Builds 26200.9278 and 26100.9278) PreviewWindows 11 Version 25H2/24H2用の更新プログラムKB5120998では、タスクバーの位置を上下左右の4方向に自由に配置できる新機能が追加されています。スタートメニューも刷新され、サイズの変更が可能になり、「おすすめ」→「最近」への名称変更や、プロフィール情報の非表示などによって、より自分好みに調整できるようになっています。その他、Windows検索の改善、ファイルエクスプローラーのパフォーマン改良、管理者向けの新機能「Administrator Protection」など多数の新機能が追加されています。リリースノートは以下の通りです。Gradual Rollout:TaskbarThis update gives you more ways to customize the taskbar to match how you work and make better use of screen space.New! You can now choose whether the taskbar appears at the bottom, top, left, or right side of your screen. In these other positions, tooltips, flyouts, and animations will still come from the taskbar, and most customization settings, such as never combining taskbar icons, will work with all locations. Search box and smaller taskbar are currently supported only for top and bottom positions. To change the position of the taskbar, go to Settings > Personalization > Taskbar > Taskbar behaviors > Taskbar positionNew! A smaller taskbar option is now available to help maximize screen space on smaller devices. This experience reduces the size of taskbar icons and the taskbar height while maintaining access to key features such as Start, Search, and the system tray. To change your taskbar size, go to Settings > Personalization > Taskbar > Taskbar behaviors > Taskbar size. When set to Small, both the icons and the taskbar height become smaller.New! The Taskbar behaviors section of the Taskbar settings page is now expanded by default for improved discoverability.This update improves reliability of loading the system tray area of the taskbar for non-touch PCs.Start menuThis update makes the Start menu more personal with several customizations.New! You can choose between a small or large Start menu, in addition to "Automatic (default)" that's available today. To customize this, go to Settings > Personalize > Start > Start menu size.New! The "Recommended" section is renamed "Recent" in the Start menu and Settings page (Settings > Personalize > Start).New! You can now hide your name and profile picture in Start by going to Settings > Personalize > Start > Hide your name and profile picture on Start.New! The Start menu settings page is now redesigned with section-level toggles to independently show or hide Pinned, Recommended, and All.Windows SearchThis update includes improvements that make Windows Search more dependable, easier to scan, and clearer before you click.New! Search home has been simplified to reduce visual clutter and make it easier to get back to recent searches quickly.New! Search does a better job showing where a result comes from—app, setting, file, web result, or Store suggestion—so it’s easier to tell what you’re looking at and where you will go before you click.New! A new setting in Settings > Privacy & Security > Search lets you choose whether web and Microsoft Store suggestions appear alongside local results.New! Windows now automatically indexes your most used folders to make those files appear in subsequent search results. You can control the setting at Settings > Privacy & Security > Search > Automatically find additional relevant locationsFile ExplorerNew! Launching File Explorer Home is now faster and more responsive.New! The Recommended section on Home now supports touch scrolling, making it easier to browse files in the carousel.General designNew! This update introduces updated progress indicators across Windows, providing a more consistent and modern experience during startup, sign-in, restart, shutdown, and update installation.Windows ShareNew! This update adds the ability for users signed in with a work or school account to discover and install relevant apps directly from the share window. To manage this experience, go to Settings > System > Share and select Show suggested apps in share surfaces.Administrator ProtectionNew! Administrator protection aims to protect free floating admin rights for administrators. It enables users to perform administrative tasks using just-in-time privileges. Administrator protection is not classified as a formal security boundary; it hardens the security against elevation-of-privilege attacks with the introduction of profile separation. This feature is turned off by default and can be enabled using OMA-URI in Microsoft Intune or through Group Policy. This feature was previously disclosed in October 2025 (KB5067036) and is now beginning to roll out.Multiple desktopsSwitching between virtual desktops is now smoother and more responsive.Display and GraphicsThis update improves the reliability of navigating to and interacting with Settings > System > Display.This update improves the persistence of previous brightness settings once the energy saver turns off.Task ManagerThis update improves the reliability of displaying app history information in Task Manager.Windows SetupThis update streamlines the experience for selecting a secondary keyboard during Windows setup, making it easier to skip when it isn't needed.This update improves the Get Started app with new App install, Site pinning, and Theme pages to help you personalize your device and discover new features.Windows UpdateThis update improves restart behavior so PCs that were manually put to sleep return to sleep after completing an update, even when automatic sleep is set to Never.App updatesThis update adds support for participating apps to coordinate their updates with Windows Update. Supported apps can benefit from improved scheduling and a more streamlined update experience. For more information, see Windows Update Orchestration Platform (UOP).Microsoft storeNew! This update provides support for in-app purchases in packaged WinUI 3 apps that require elevation, helping developers offer purchases through Microsoft's integrated commerce platform.Windows Autopilot device preparationNew! Device association for Windows Autopilot device preparation is now generally available. This feature helps organizations identify trusted devices before enrollment, enabling device-targeted policies, automatic corporate device enrollment, and additional setup customization during the out-of-box experience (OOBE). To learn more, see Windows Autopilot device preparation.Microsoft Execution ContainersNew! This update introduces Process Isolation for Microsoft Execution Containers (MXC), a fast, lightweight boundary for responsive workloads such as coding agents and model-generated code. It uses Windows operating-system containment capabilities to restrict access to resources such as files, networking, the user interface, and other operating-system capabilities according to policy.Agentic processesNew! This update introduces preview platform support for tagging agentic processes. Authorized components can assign an opaque agent identifier to a process token. Windows protects the marking and automatically passes it to child processes. When an agentic process authenticates through Web Account Manager (WAM), WAM includes the agent identifier from the process token in the authentication request. As a preview capability, the identifier format may change in future releases.CryptographyNew! Post-Quantum Cryptography algorithm ML-KEM can now be used as a standalone algorithm for TLS key exchange, in addition to hybrid groups added previously.NetworkThis update improves resiliency and recovery from potential VPN process-related background that can cause processes to stop responding.This update improves the output of netsh wlan show wlanreport command for Japanese users.InputThis update improves the reliability of typing with the Japanese IME by reducing potential hangs.This update improves the persistence of voice typing settings for automatic punctuation and launcher.Windows Management Instrumentation Command-line (WMIC)Starting in August 2026, Windows 11, version 24H2 and 25H2 will no longer include the Windows Management Instrumentation Command-line (WMIC) utility. WMIC is already removed by default in new installations of Windows 11, versions 24H2 and 25H2, and will no longer be available as a Feature on Demand (FoD). This change affects only the WMIC utility; Windows Management Instrumentation (WMI) remains supported. Learn more: Windows Management Instrumentation Command-line (WMIC) removal from Windows. Normall Rollout:AccessibilityThis update improves the Natural Voices experience in Narrator, helping Natural Voices install and set up successfully from the settings dialogAppThis update improves Japanese text input in RemoteApp applications, helping text appear correctly during typing.OutlookThis update improves search in classic Outlook on Japanese-language devices, helping searches in Sent Items and Advanced Find display the correct "To" label and return expected results.PointerThis update improves touchpad reliability on some devices, helping touchpad gestures and mouse-drag actions respond as expected.既知の不具合更新プログラムには既知の不具合は存在しません。更新プログラムはWindows Updateの他、Microsoft Update Catalogを利用してインストールすることができます。