If the ContStim video server is not displaying an image
properly (it will typically give an error with incorrect 
frame rate),

Go to Display properties (right-click on the desktop)
Settings
Advanced
Adapter
List all modes
Select a mode with 120 Hz, e.g. 1024 x 768, 256 colors, 120 Hertz
OK
Apply
OK

If the monitor shows an error now, turn it off and on.
It should now be displaying Windows at 120 Hz.  
ContStim should now run fine.

To get all the colors back, go to 
Display Properties
Settings
and change Color Quality from Low (8 bit) to Highest - 32 bit.
Apply
OK

The monitor will now be displaying Windows at 85 Hz, 
but it shouldn't have any trouble switching into 
120 Hz mode for ContStim.

-- Matt