Thanks. I don't mean by inverting pictures, i mean inverting the overall colors of the computer.
Don't worry, I get what you mean. See, I was thinking you could invert your background, then replace your icons with inverted versions (IconoMaker is a good free program that I use). Then you could use the Display Properties to adjust the look of your folders, taskbar, windows, etc.
Like patio said, something that does this automatically might be a drain, but still, I'll be on the lookout for something.