One of the things I have to do as a writer is to take countless screen shots of Windows and various applications. One way to do this is to press the PrintScreen (PrtScrn) button on the keyboard to copy the screen to the clipboard, then you can paste it into a paint program or photo editor and save it. Now at last in Windows 8 there is an option to take a screen shot and save it directly to disk.
No matter what program is running or what you are doing, holding down the Windows/Start key and pressing PrtScrn key saves the current screen to a folder called Screenshots in the Pictures Library (actually the My Pictures folder). Each file is called Screenshot (x).png where x is a number. Windows automatically increases the number each time.
My heavy usage of screen shots probably isn't typical, but there are some general uses. For example, if you are playing a game or using an app and want to save the screen and email it, or post it on Twitter or Facebook, then Windows+PrtScrn will do it.
You might expect Windows to look at the number of the last screenshot taken and add one to get the next number. It doesn't though, the number is stored in the registry and incremented every time. If you emptied the Screenshots folder and wanted to start again with Screenshot (1).png you need to reset the value in the registry.
Press Windows+R to open the Run box and enter regedit to start the registry editor. In the left hand pane navigate to HKEY_CURRENT_USER \Software \Microsoft \Windows \CurrentVersion \Explorer and select it. On the right is a value called ScreenshotIndex. Double click it and enter 1 (or whatever number you want the next screen shot to be).