I have a notebook (HP 530) and I always noticed that with the AC power cable plugged the brightness of the monitor is greater than without it (i guess its to save power when running on battery). But today I notice that there's no change if I plug/unplug the wall power cable, the brightness is always as if the AC power cable is plugged even when it's not.
THe laptop is running Win XP, and on Control Panel > Power options, I dont see anything that could configure this setting.
What else can I check?