Windows 7 Problem with the brightness of the screen--reverse dimming???

I'm having a problem with my screen brightness setting. I'm currently using a (very old) dell latitude e6400 with Windows 7 while my normal laptop is being repaired. The battery is shot, so it only runs for about 30 seconds without being plugged in. A few days ago, the screen went extremely dim for no reason. I changed the brightness setting, the power plan, adjusted absolutely everything I could, and nothing helped. Its about as dim as the lowest brightness setting would usually be, and yet all of the computer settings say it is at full brightness. I figured it was just an old, broken computer, and Id just have to deal until my regular one is fixed. Then today I unplugged it, and all of a sudden it was bright again! It turns out that when it is unplugged, the brightness works just fine! I have no idea what is happening or how to fix it. Its also not tat the power plan is set to be dimmer when the computer is plugged in, I checked that and it didnt help. Any suggestions? Or is this computer just a lost cause?
(Also, I apologize for any spelling/grammar errors, a few keys on this keyboard decide not to work sometimes including te apostrophe)