Carol,

There are a number of causes for this but you might try adjusting the monitor settings which can normally be adjusted from the front buttons or knobs on the monitor. You should be able to adjust the brightness and contrast from there.

Next, try adjusting the "screen resolution" and "refresh rate" like this:

RIGHT click on a blank area of the desktop screen, choose "Properties" from the menu that pops up. When that loads, click on the "Settings" tab. When that loads, move the "Screen Resolution" slider to something like "800 X 600" or "1024 X 768", whichever is better viewing for you. Now click on the "Advanced" button, then the "Monitor" tab, and change the "Refresh Rate" to something no less that 85 Hertz. (Assuming your monitor will handle that rate. The larger the number of hertz, the more resources will be used, so use what works for you.) Now click on the "Apply" button, then OK, then "Apply", then OK.
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If none of this helps, you should try downloading and installing the newest drivers for you video card to see if that helps. The drivers can usually be found at your computer manufacturer's website.

Hope this helps.

Grif