How can I reduce the screen brightness under Windows 7? (21.5" & 24" iMacs)

Thanks, I've already tried this, but it's the same adjustment range
as when using the brightness keys (F1/F2) - the minimum is far too bright.
Only in Windows -- not in OS X!
Hence I was looking for a special program to get it darker.

Thanks for your helpful answer.
It is a bit darker now, but it's not enough.
White regions are still too bright.
It's still unpleasant to read black on white text
in the evening or night on the screen.
Is there another software to get it darker...?

Thanks again!
I've tried it, but unfortunately I find it not ergonomic
to read bright text on black background.

Then I turned off Aero and via "Advanced appearance settings..."
I tried to define colours for "Window Text" background etc.

This is not perfect (some regions can't be defined at all),
but it's a little relief. On certain regions or when playing a DVD
for example this method has no effect on the brightness.

In my opinion there is an error in the Boot Camp driver
which is responsible for the insufficient adjustment range.
At least the 21.5" iMacs have a perfect range under OS X.

Unfortunately that doesn't help for games, so I'm
still searching for a software solution. Any idea...?