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.
To check if you have this driver already:
- Open Windows Device Manager and see if the micro:bit USB device is shown. If it is...
- find the mbed Composite Device entry, right click on it and select Uninstall device.
- A new window will open, mark the Delete the driver software for this device box and click Uninstall.
- Unplug and replug your micro:bit and it should appear under USB devices.