Ow. First of all, please tell us which flavor of Windows you have (sorry if that's a wrong assumption, but it's the best shot in the dark).
Has anyone else (even a cat) been using your computer?
Let's assume 7 (or maybe Vista, or XP). Go to your start menu (button in leftest bottomest corner of screen). Find Control Panel on the menu that opens there. Make that open up. In the Control Panel, look for Display (in Vista, it's Personalization, and at this point I can understand why some people hate Vista).
Here you will find options for general font size, but also check screen resolution. If the actual icons on the desktop have changed in size (not just the font), I would suspect resolution. Change it to something smaller (lower numbers) if you can.
More questions and answers would be helpful at this point.
Please use your browser's View menu to change browser font size if possible. (I hate to shout lol)