Windows 7: Brightness and power plug in

I don't know if this is the right place to post this and I don't know if the problem's been solved already.

I'm using a Dell XPS M1330 laptop and I have a strange problem. The brightness has a certain interval in which you can adjust it. At maximum it is quite bright and the lowest is dark but still visible. But when I plug in the power the interval is 'shifted downwards' so to speak. What was previously low or middle power is now max, and the new minimum is almost completely dark (you can just barely see white surfaces on the screen).
It hasn't always been like this and I don't know what changed it. Some update I guess... I haven't been messing with drivers or anything manually.



I'm aware that the interval of the brightness adjusts to whether you're high performance, balanced or power saver and I _have_ taken this into account. I've been using linux gentoo previously so I'm no retard....