>>an nVidia GeForce2 MX400 (no laughing!)>>
One served me well for a number of years...:-)
Try right clicking on the Desktop>Oroperties>Settings>Advanced Button>Adapter>List All Modes and check that your chosen resolution/colour/refresh rate is supported by the card.
You can change the setting from here if you wish (ensure that Change without Reboot or words to that effect is Enabled) by highlighting and clicking on the selection.