but doesn't now, then the fear is this may be a problem with the video card.
Vista should automatically detect the 2nd monitor, but to use it, you would have to do this.
Goto the Control Panel, (from the Start menu), then find the Display icon and run that. In the new page, look in the left hand column for "Change display settings" and click that.
In the next new page that displays, if you can see 2 monitor images showing, marked as 1 and 2, then check that the other settings are OK.
If only #1 monitor is showing, then click the "Detect" button.
The settings that I have on my dual monitor setup are;
Display: Pull down menu shows the make and model # of the monitors
Resolution: Click the first image, 1, and this should show the Recommended resolution. Any 2nd monitor, 2, may be different.
Orientation: Normally Landscape.
Multiple displays: Pull down menu, normally "Extend these displays" is selected, but you have other choices.
If monitor 2 displays but still doesn't work, highlight monitor 2, then click "Advanced settings", then in the new window select the "Troubleshoot" tab. Click the button if it is not greyed out.
Let us know what you see, but be prepared for the worse.