Here's a weird Mac-ism that I found out only after I toted my iMac into the Apple store, where a "genius" actually diagnosed and fixed the problem in 1 second.
My problem was, the screen was completely washed out with some important programs, like Word. Some of the icons were so broken up (white), that you couldn't make them out at all. I'd looked for solutions on the net, tried everything suggested, and finally took it into the store.
As it happens, if you want to change the appearance of the screen, you go to Preferences, then Displays. However, if you want to change the *contrast* of the screen, it's not there, but it's down under "Universal Access," which is meant to deal with a variety of vision problems and other handicaps. Sometimes, apparently, the iMacs ship with the contrast pushed to the high end. I don't know why that slider wouldn't be under Displays, but it's not.
If your screen seems washed-out, check the contrast under Universal Access.