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Click on the

Start Button > All Programs > Accessories > Command Prompt copy and paste this into the command prompt and press
Enter what is in Bold Red including last quote mark that should start the default Aero theme.
rundll32.exe %SystemRoot%\system32\shell32.dll,Control_RunDLL %SystemRoot%\system32\desk.cpl desk,@Themes /Action:OpenTheme /file:"C:\Windows\Resources\Themes\aero.theme"
Jerry