I am not familiar with your particular monitor, but check out the controls. Having adjusted the picture to fit the mask, you may have to tell it to save the settings before exitting from the monitor setup menu. Did the monitor include a setup floppy disk? Might be worth running this through...
Also, some video cards have built-in facilities for adjusting picture centring - goto SETTINGS, CONTROL PANEL, DISPLAY, and check out the tabs for one that might give you additional settings.
ROGER - GØAOZ.