Isso pode resolver seus problemas:
"Auto dim or also referred to as Smart Dimmer in Windows 7 is available in computer systems that have the hardware which supports this feature. Please see the troubleshooting steps below that would help resolve your issue.
- Open Power Options by clicking the Start button, clicking Control Panel, clicking System and Security, and then clicking Power Options.
- Under the current power plan, click Change plan settings.
- On the Change settings for the plan page, click on Change advanced power settings.
- In Advanced settings, scroll down the menu, click on Display and see if you find the dim feature listed here. You may change the settings for Auto dim if present under display, accordingly to as you want."
O texto acima é citado de Microsoft Answers: Desativando o escurecimento automático do Windows 7 na área de trabalho