If the Mbed Serial driver has been manually installed (by downloading and executing the .exe) it might interfere with the Windows 8/10 automatic installation of the WebUSB driver. It is possible that a driver was previously installed on the Windows machine if you have upgraded from an older version of windows.

To check if you have this driver already:

  1.  Open Windows Device Manager and see if the micro:bit USB device is shown. If it is...

  2. find the mbed Composite Device entry, right click on it and select Uninstall device.

  3. A new window will open, mark the Delete the driver software for this device box and click Uninstall.

  4. Unplug and replug your micro:bit and it should appear under USB devices.