I would have thought the monitor would automatically play a 4:3 game with black bars on the left and right, kind of like how a TV automatically does this, as a signal is sent with the TV broadcast saying what aspect ratio to display.
I saw my mate playing a game on a widescreen monitor, and it was all horribly stretched out. I'd imagine the monitor probably has a manual option to switch to 4:3 - maybe try the buttons on the front of the monitor. Otherwise who in their right mind would ever buy one? Older games probably weren't designed with wide monitors in mind.
As far as i can gather from the forums, there's no sure way to "automatically" detect what aspect ratio the monitor is running through the computer- if you code a game, you should stick in an option for the user to manually specify whether they're using a 4:3, 16:10 or 16:9 monitor. I am choosing to use a 3:2 aspect ratio, and there are black bars on all monitors.