Replying to Need help with my new mac!

Well actually my grandma's mac, but i guess that's irrelevant.

Anyhow, i have a problem. The screen seems to have grown and now the whole thing doesn't fit in the monitor.  Its hard to explain, it never shows the whole thing but it follows the mouse around.  Like when the mouse is in the top left corner, the bottom right corner of the screen can't be seen, and so on.

I tried changing the resolution and i got no positive results.  Help me please, i need to give the computer back by tomorrow.