Flashing display after refreshing performance index in System

In the System Control Panel applet, I tried refreshing the performance index
for the first time. Message said that screen may flash during the process.
Both LCD monitors began flashing on/off about once a second and kept doing
so. Rebooted and they resumed flashing! Cannot stop this flashing - system is
unusable. Any suggestions would be most appreciated.

New Vistal installation, core 2 quad, 4G, nVidia 8800 GT graphics with two
LCD monitorrs.