ZoomIt is a fantastic tool for presentations. You'll find it at:
But I've always battled to get it to display exactly where I want and tracking around the zoomed screen is a nightmare.
Then I figured out that before you hit "Ctrl / 1" to zoom, put the cursor at the middle of the section you want to zoom.
You can make minor adjustments by moving the mouse, use the trackwheel to zoom in / out, click the left mouse button to "stabilise" and then you are in the "Ctrl / 2" window by default so "Ctrl / Shift" will draw an arrow etc.
And it would be remiss of me not to point out that you can do this in Windows via the built-in commands. In a browser, type "Windows key" and then "+" to zoom out and then "Windows key" and then "-" to zoom back in. This is in fact the Magnifier tool.