Do you mean the windows task bar (start, etc)?
I found a prog called WatchCat that is able to completely hide the taskbar, google search for it, I dont have it bookmarked here at work
But I always had the problem where the bottom status line of advmenu was cut off half-way
device_video_output auto is the default setting and apparently it was picking a window, when i changed it to fullscreen it really went to full screen