You say you have full use of the Smartboard and it displays what is on your screen.
So, you have both your computer's monitor and the Smartboard hooked up and they both work?
Going back to Display properties > Settings, in the little window above where it says default monitor, can you see two monitors displayed (one will be fainter than the other), or is there just one?
If two, highlight the 2nd one.
Or, where it says "default Monitor on NVIDIA GeForce 6150SE nForce 430", can you 'pull down' the list for that window to select the 2nd one?
From what you say everything is working fine and your terminology is right, Extended Display or Extended Desktop both work for me.
I'm just checking mine, but I am on Win 7 so the dialog pages/windows are different. But on my equivalent of Settings, I see a "Detect" button. Does your XP have that?
Alternatively, I wonder if your NVidia video card has it's own control panel for video and monitor settings. I know my older Dell XP system has. I can right click the Desktop and there is an NVidia option there, or I also have a Startup folder and shortcut for NVidia.