So, the Laptops graphics card is fine, it is a ATI Radeon Xpress 1100 with shared VGA memory.
It has a compatible S-Video output which is switched with "Function" F4 as you have stated.
By pressing and holding the "Function" key and press F4 you should toggle between laptop screen and external display or dual display.
You previously stated that when you hit "Function" and F4 the laptop screen did not change, this may be because it is in dual display mode which is the default.
Check the AVANT has Camcord on the display and if it does not then bring it up from the Beo4. Normally it would change automatically when a signal appears on the S-Video input. Is Camcord on the AVANT display ?