Before delving too deeply into the troubleshooting process, make sure that the surface of your monitor is clean. You might be surprised by the resulting improvement in image quality. Clean your monitor using a lint-free microfiber cloth moistened with a little distilled water.
Is the entire display blurry, or is it just the desktop wallpaper? If your background image is blurry but the text is clear, the resolution of your desktop wallpaper is too low for your screen. When the wallpaper is expanded to fill your screen, it becomes blurry due to the quality of the original image.
Are you using an LCD display? Although LCD monitors are superior to old-fashioned CRTs in nearly every way, every LCD screen has what is called a "native resolution." At any resolution other than the native resolution, the image will look blurry because the monitor will scale it to fit the whole screen. Check the instruction manual for your monitor to learn the native resolution, and run your display at this resolution whenever possible for best quality.