I have done clean installation of Windows 8.1 on my HP Pavilion G4.
Then I realize that I cannot put my computer on sleep mode and cannot adjust screen brightness.
I understand that those problems caused by the graphic card driver.
My computer is running on AMD Radeon HD 6400 series.
Thus, I manually search on internet and found a latest graphic card update on microsoft site that is claimed compatible on windows 8.1 (link: http://msdn.microsoft.com/en-us/library/windows/apps/dn344644.aspx).
I've upgraded it into the latest one. I've check on Device Manager, the driver is running well.

But my problem did not solved!

Please give me the solution!

Hi,

How about go to Device Manager, then uninstall Graphic driver and restart your computer, system will install its build in graphic drver. You can have a try.

In addition, access to HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Brightness and set its value to 100 for test.