Hi , display settings are usually a combination of OS settings and graphics card driver settings.
I don't know what OS you are running but on XP you can right click on the desktop and choose properties to bring up the Display settings window, got to the settings tab and you can adjust your monitor resolutions and which is the primary monitor ect.
Your graphics card driver shouls aslo have software that allows you to set more advanced settings like whether you have dual view or horizontal span or just clone.
Nvidia cards have nview where you can send apps to one monitor or another if you are in dual view or you can just drag the window to which ever monitor you need.
If you are running a Mac then I have no clue.