Katsumi,
Thanks for posting to the forums with your question. We have created support ticket number 00385158 to support your issue if you want to call us about it. If you want to do this, please contact us on the phone line listed at www.prometheanworld.com/.../technical-support-numbers.
We have not heard back to hear if John's system is working properly yet, but we can begin to help you. When you connected the Activboard to the computer running Windows 8.1, do you have an errors on any devices in the Windows Control Panel, under Devices->Human Interface Devices? Look for any yellow exclamation points in that area, and let us know if errors exist.
Also, are you connecting directly to the computer, from the board? Or are you using a wall-box, or termination box of some kind, or connection panel on a stand? We need to know if the USB cable goes directly from the board, to the computer, or if there is an intermediary connection that it passes through.
Please let us know the results of this troubleshooting by replying to this post, and we will do our best to solve your issue. Please let us know if you have any other questions.
Thank you,