Windows 10 build 9841 (fbl release)

Windows 10 build 9841 is the official Technical Preview build of Windows 10, demonstrated on 30 September 2014 by Microsoft at a closed press conference and publicly released the next day on 1 October. It is the first publicly available build of Windows 10, released to users who joined the Windows Insider Program.

Media Center
If you add Windows Media Center to this build using a purchased Windows 8.1 product key, the system subsequently identifies itself as running "Windows 8.1 Pro with Media Center", along with a warning that the OS has not been activated.

Window thumbnail color error
If window auto-colorization is enabled, the window thumbnail borders on the taskbar do not change color even after the wallpaper changes. This issue can be remedied by manually changing the window color through the Control Panel.