i9100/Vista: Screen gets darker after reboot and login

I've had this problem ever since I installed Vista on my Inspiron 9100. After a reboot the screen is nice and bright and stays that way until around 10 seconds after login. At this point the screen brightness drops to around 75% brightness, however it seems that it's dropped the maximum brightness level as messing with the settings in as many places as I can find doesn't allow me to make it any brighter. I have a Radeon 9800. Has anyone else seen this and know how to fix it? Steven