can't get lighting right

[viking1066]

Just got the Demo and I accidentally took the Gamma up all the way to the maximum, now it won't switch back. I press up down, whatever, it is completely locked. I tried erasing the Demo and reloading it but it still remembers that the Gamma is all the way up. I really want to try out the game but it is so bright that there is only a white screen.

Thanks