You can't hide individual ones, but you can hide the entire taskbar (what you call the startbar is actually the 'Taskbar' )
Windows XP:
To hide taskbar manually
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right click an empty part of tasbar and 'unlock' it by removing the tick from 'Lock the taskbar'. Now move mouse-pointer over top-edge of taskbar till pointer changes to a double arrow. Left-click and 'drag' to bottom of screen. Do same to drag it back into view.
To 'auto-hide' taskbar
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leave taskbar in-view and locked. Left-click Start button and select 'Properties'. Click 'taskbar' tab. Enable 'Auto Hide Taskbar' then click 'OK'. Taskbar will now only appear when you move mouse to bottom of screen, and disappears when you move mouse away from it.