Many system administrators prefer to have OpenVPN start automatically during system startup. In order to force OpenVPN to start automatically on boot you need to modify the service in Windows.
The OpenVPN Windows installer will set up a Service Wrapper, but leave it turned off by default. To activate it, go to Control Panel > Administrative Tools > Services, select the OpenVPN service, right-click on Properties, and set the Startup Type to Automatic. This will configure the service for automatic start on the next reboot. See the screenshot below for details.