If you are running Windows Vista, you might have faced issues in 'Print Screen' or 'Screen Capture' functionality.
In other version of Windows by pressing key 'print screen' or 'Alt + print screen', you can take snapshot of your entire screen or active window, respectively. Then you can paste it into 'Paint' and save it. But in Windows Vista, pressing key 'print screen' or 'Alt + print screen' does not capture the screen. I think they have modified key combination for screen capture.
In Windows Vista you need to press 'Fn + print screen' or 'Fn + Alt + print screen' to capture the entire screen or active window.
In most cases, 'Fn' key is located at bottom left part of keyboard, between 'Ctrl' and 'Window' key.