iMac Display Brightness Problems

Hi all. It's me again with ANOTHER problem

The more avid members of this message board may know that I've been having loads of problems installing XP via bootcamp (when I go to install XP, the screen goes black, but I can still hear lots of CD Drive activity)

OK, that isn't the problem, but I think I've just found a problem that may be related....Basically, I can't change the brightness of my 20" iMac. I have two choices: 100% on, or totally off. If I press F14 to dim the screen by one notch, it just goes black. The same goes for the slider in System Preferences. If I move the slider just a smidge to the left, the screen goes black. I have all the latest firmware & OS revision (10.4.8), I reset the PRAM and I've even SuperDupered my drive & done a fresh re-install, and the problem still exists.

I mentioned the XP bootcamp problem first because I'm thinking that I can't install XP because maybe the screen gets dimmed when you install XP (and because my display can't get dimmed, I cant see any installation instructions) I hope that made sense!

So basically, I really need your help. If I can fix the display brightness, then mabye I can fix the XP problem as well....this has been driving me mad for the last month, so someone please put me out of my misery!