It can be both!
You have to stick your nose in Control panel and in your video card panel
If its XP, then go to Accessibility Options in Control panel, then go to the Display tab and under High Contrast there is something like click Settings or a current high contrast scheme list. Choose from list or uncheck the use high contrast box! (You probably accidentlly pushed a few keys togheter activating it...)
If it's a video card problem, go to your Video card control panel (if you disabled the control panel then right click desktop-properties, settings , advanced , and you'll see your video card on a tab. Click it and it will open...) and check for something related...