Often if you are holding a presentation to showcase a program, the audience simply can't see which button you have current clicked. A normal button is 32x32 pixels high and even on a 9x9 meter canvas, it's hard to spot.
A great zoom utility is ZoomIt by the SysInternals guys (now also Microsoft) as I found out today. I note this utility here because it's not only very simple to use but you can also draw on the zoomed area with your mouse to easily highlight a button or function. And if this is not enough, you can also type text into the zoomed area.
Best of all, it's free.