The alternator could be undercharging or undercharging. If the guage is supposed to be in the middle, and it's at the 3/4 mark, it could be overcharging, which would mean that the voltage regulator would need to be checked.
Usually, if the lights dim when doing all this, it's the alternator not putting out enough charge. The lights may dim just enough for you to notice when the brakes/windows are applied, but they shouldn't dim so much that you can't see anything.