Every monitor has a maximum refresh rate it is capable of displaying at a given resolution (for example, a typical 17-inch monitor of a few years ago will happily crank out 85Hz or more at a resolution of 800x600, but may only be capable of 60Hz at 1600x1200).
If this refresh rate is exceeded, the image will be distorted and unusable.
Older CRT monitors, especially 15-inch or smaller ones, tend not to be capable of a refresh rate much over 60Hz, especially at resolutions greater than 640X480. Also, setting the refresh rate does not have the same effect on LCD or flat panel monitors, as their screens are not redrawn in the same way.
To change your monitor refresh rate: Right click on an empty space on your desktop (no icons) and select 'properties.' Click the 'settings' tab and choose the 'advanced' button. Now choose the 'adaptor' tab and click the 'show all modes' button.
This presents you with a list of resolution, colour and refresh rate options that your video card\monitor combination is capable of displaying. You can experiment with these settings by using the 'apply' button. Note that a refresh rate of 75Hz or above is generally recommended, (keep in mind most monitors do not support settings higher than 85Hz).
After you set the refresh rate, you may feel a little uncomfortable for a short while as your eyes adjust, but rest assured you will grow to appreciate the difference in ease of viewing.