RIGHT click on a blank area of the desktop, choose "Properties" from the menu. When the Properties window loads, click on the "Settings" tab, then click on the "Advanced" button. When that loads, usually, you can click on the "Monitor" tab. You'll see the refresh rate drop down arrow that allows for changing...On older operating systems, "Optimal" was a good setting to choose although specific levels could be chosen. On Windows 2000 and XP, a specific number should be chosen. Setting is too high stretches resources...while setting it too low will cause a flickering screen. Usually, between 75-85 works best for most.
Hope this helps.
Grif