Make, model and operating system ?
Is all text like this ? System tray, clock etc. If you right click the desktop etc is the menu blurred ? If so it perhaps suggests a driver or screen issue.
What about when you first do a cold start and any splash screens/bios messages etc messages appear. Are they blurred as these load before anything Windows/driver related ?
What's it like if you boot into safe mode ?
Have you tried a system restore back to when it was OK ?
Without actually seeing something like this it's a bit difficult to say for sure.