http://superuser.com/questions/951531/windows-10-upgrade-lost-brightness-sony-vaio-laptop
Do this:
I had to edit the registry key FeatureTestControl in
HKEY LOCAL MACHINE\SYSTEM\ControlSet001\Control\Class{4d36e968-e325-11ce-bfc1-08002be10318}\0000 (or 0001)
I set the value to 0x0000fb20 (64288). Screen brightness works fine now.
