Video/Picture Adjusts Brightness/Contrast Automatically

I own a Dimension C521 desktop computer that originally runs on Windows XP but now runs on a Windows Vista (was upgraded the time Vista came out). Five days ago, the computer's picture started looking too bright and the contrast makes everything viewed on the monitor very difficult to read. When I turn on the computer at first, the picture looks fine but when it goes to the welcome screen, it would suddenly flash a "Auto Adjusting" message and then the whole computer's color would change to something unreadable. I have to go on NVIDIA Control Panel to change the brightness and contrast each time before accessing the computer. 

Is there a way to fix this problem? I know this has nothing to do with the monitor since sometimes, the colors looks normal until it decides to adjust itself. 

Thanks very in advance!