i installed windows 8.1 RTM BUT when i start download anything using IDM few seconds windows show my lock screen although i adjust my power options to NEVER (TURN OFF DISPLAY OR PUT COMPUTER TO SLEEP ) WHAT IS THE CAUSE OF THIS PROBLEM ??
Seems to be an driver problem of the display card! I had a bit similar on Server 2008 with an not fully supported Display Card, an ATI card. Open the Device manager and check what Display Driver is installed.
I also have this graphic card in my Win7 laptop and I never had this problems, but try reinstall/update driver. If it's not a problem, check Power Options.