Screen dimmer / brightness reducer

I'm wondering if there is software that allows you to manually and quickly reduce the brightness of your monitor.

I understand that I could manually reduce brightness with the physical keys -- but that takes far too long with the menus and sub-menus on my monitor (Samsung 2333sw).

I also understand that software that dims or reduces brightness may not actually reduce brightness (i.e., power output), but may only make the screen darker. I need it to reduce eye strain when I'm working in a dark room.

I tried an application called "Power Dimmer," but it doesn't serve my purposes. It functions as a screen saver. In other words, it won't work if I'm using the computer.

Final note: I know that this program exists for OSX. It's called ScreenShade, and does exactly what is described here. Anyone know of an option for XP?