If you use a laptop for presentation, you are probably hunting for the FN+F8 key combination or other manufacturer shortcut to change the display mode. If you use a desktop for presentations you are most likely delving into the graphics card functions to find the display options.
But if you have Windows 7 (laptop or desktop), skip all the hunting and just click the Windows key + P.
Win+P brings up a dialog that makes it easy to switch to Mirrored or Extended Desktop for presentations.
– Troy @ TLC