By default Sandboxie is configured to load and start automatically. To have Sandboxie load only when you need it, make the following changes.
In Sandboxie Control, open the Configure -> Shell Integration window, and clear the checkbox When Windows starts to stop Sandboxie Control from starting.
Open the Windows Services configuration window: Start menu -> Control Panel -> Administrative Tools -> Services. Then locate the Sandboxie Service. Double click to bring up its properties window. Set its Startup type to Manual rather than automatic.
The driver component of Sandboxie is started by the Sandboxie Service. Therefore, setting the service to start manually, indirectly also sets the driver to start manually.
Starting Sandboxie Control will also start the service.