1. Downloading UltraMon, http://www.realtimesoft.com/ultramon/
2. Make a shortcut to XBMC.exe if you don't already have one. Place it on your desktop (or wherever). Right-click on the shortcut select properties.
Set the environment variable SDL_FULLSCREEN_HEAD=2 (2 for secondary display / 3 for third display etc.) apply and exit...

example: "C:\Program Files (x86)\XBMC\XBMC.exe" SDL_FULLSCREEN_HEAD=2

3. One's again Right-click on the XBMC shortcut. As you can see UltraMon have added two new tabs, "UltraMon - Display" & "UltraMon - Window"
a) Click on "UltraMon - Display" check "use custom settings" choose resolution for "secondary display" ex: 1920*1080 and check as enable.
b) Click on "UltraMon - Window" check "use custom settings" show: [normal] on monitor 2 (or whatever display/monitor you like but HEAD="x" must be the same) apply and exit...

4. Launch XBMC, change resolution to AUTO, test resolution. XBMC will automatically choose the highest resolution possible for that display.
a) Now select 1080i (this will make the settings stick) and your done!