Is there any particular reason you don't want xbmc to open on startup of the computer?
If you're running windows, you can use the free program AutoHotkey to assign a joystick button or combination of buttons to open xbmc.
You'll need to have AutoHotkey load when the computer starts and edit the script. More info here:
http://www.autohotkey.com/docs/Tutorial.htm and
http://www.autohotkey.com/docs/misc/RemapJoystick.htmFor example, a AutoHotkey script of:
Joy10 & Joy11::Run mspaint
will open paint when joystick button 10 and 11 are pressed at the same time. If you replace "mspaint" with the executable file for xbmc, which I will assume is "xbmc" then the script should open xbmc when button 10 and 11 are pressed. You can pick whatever button or combination of buttons to use.