Considering the printscreen is normal, I would say it's a LCD Monitor driver issue, or even a monitor issue.
Do any other programs have this problem?
What you can try is to create a small image in paint (a 1x1 image is enough, but any image will do) paint it all black (use floodfill), save it, and then make it your background, if it is still that bright, then it's the LCD Monitor driver or the monitor itself.