Some people are more sensitive to lower monitor refresh rates than others. Go to the display control panel and click to the Settings tab. From there, click the Advanced button and go to the Monitor tab. From there, you can adjust your screen refresh rate.
As a rule of thumb, higher refresh rates are less likely to cause eyestrain.
If you pick a refresh rate and your monitor goes all blooey, don't panic. Just wait fifteen seconds and the system will switch back to the original settings. (If you have a Plug and Play monitor, it's supposed to report the refresh rates it supports, but sometimes that doesn't work.)
Note that not all refresh rates are available at all screen resolutions, so if you want to crank the rate higher, you may have to sacrifice resolution.