How to make zoom at 100% (default state) in Windows 7?

It depends on the application. In browsers such as IE, Firefox, and Chrome, you can press CTRL+0in order to reset the zoom. To zoom in, CTRL++ will work fine and CTRL+- for zooming out. The exact options will always be available under the 'View' menu, or similar.

If you are trying to change the resolution of your screen in Windows 7, this is a different story. Here is the link to the Microsoft page that tells you how to do that. Or you can just right click your desktop and go to Screen Resolution.

This is an application-specific option and in Internet Explorer, you can use CTRL+0 to make the zoom level 100% (default).