You can bypass this countdown by following this info.
Open up Control Panel > System > in the left panel click on "Advanced System Settings". In "Startup and Recovery" click on "Settings". In "System Startup" select the OS you want to start Up as default, then remove the tick from the box below.
An alternative at this point is to reduce the time in the "Seconds" box.
This does not remove the old OS but does give you a way to bypass the countdown.
Others may be able to help you to remove the OS.