First off, make sure your current theme isn't what's causing this problem for you. It is possible for themes to change the color scheme of your computer to all sorts of strange combinations.
Second thought would be that your gamma settings got kicked up. That can cause things to seem "too bright" and discolored.
Third thought is that your monitor is on its way out. Might be something simple like a bad cable, it might be something more serious. If the other suggestions don't pan out, now might be as good a time as any to consider investing in a nice LCD monitor if you don't already have one.
I have a Dell Optiplex GX110, Windows XP and I am having trouble with a discoloration on my screen. The lettering is green instead of black and the other colors are a really bright pastel colors. Can someone help me ge the right colors back onto my screen?? Whenever I print things, they print out just fine.