This may be the screensaver. If so, right-click on your desktop, click "Personalize" and then in the next screen at the bottom right click on Screen Saver. There you can choose whether to have one, how long the inactivity period is before it comes on and whether you see the log-in screen on resume or it just goes back to what you're doing.
To access other power-saving options and features, you don't have to use the control panel. Simply click on the Windows button and type "Power" or "Power options" into the search box and it will bring up these menu options in the search results for you.