This is not currently possible without using third party apps. While windows allows you to change the short-form and long-form of your default times (Control Panel -> Region and Language -> Additional Settings -> Time -> Short: hh:mm:ss tt), this change does not take effect on la barre des tâches clock.
As @CharlieRB noted, vous pouvez utiliser the Clock gadget to display the clock on your desktop, or vous pouvez cliquez sur the time to see an expanded calendar/clock flyout with the time in seconds.
