It's like the radio in your car that automatically adjusts its volume to the cars speed or environmental sound.
As far as I know every graphic card manufacturer offers a GUI-based program to change the settings.
But I've never seen a command-line based interface or a published API (application program interface, like a description of some dll-routines used to change the setttings in the hardware of the card, the running driver or whereever it stores it's data). And you surely need one of the two (either a command line interface or an API) to do what you want. Once you have it, it's a relatively easy task to write such a program (of course, it should have it's own settings to change from summer to winter and from sunny to cloudy).
Maybe better turn to the support (tech support, discussion forum) at www.nvidia.com or www.ati.com, whatever your video card is.
The usual way to adjust brightness, of course, is done with the buttons on the monitor. You can't automate that. It surely must be done on the PC.
I was wondering if there is a way for your computer to automatically change the screen brightness with the time. Like you set up the brightness you want for a specific time and it automatically changes. It can be anyway; hardware, software, whatever. If there is a software solution it needs to be compatible with Windows XP and/or Vista.
Thanks in advance.