old thread but i've just been having issues with this, still haven't come with an acceptable solution so far.. (Windows)
set the secondary monitor as the primary screen
close xbmc if i have it open
launch xbmc and set as fullscreen (bsckslash in windows)
The only problem now is that as expected any new applications opened will pop up on xbmc's screen, there's also the check for admin rights that takes over both screens but i expect this is just windows being rubbish.
If anyone has a better solution running in Ubuntu please let me know as that is my main OS