The video card installed in your system looks compatible to 'Windows 7'. Good that you updated the display drivers and getting display on both monitors. It indicates that the 'card' detects and sends a healthy signal to both screens.
You may follow the steps listed in the 'Microsoft article' to extend the display: http://bit.ly/9juCeA
Alternatively, you may also use the 'nVidia Control Panel' to configure the dual-display as per your choice. You may find these steps useful: - Right click in a blank area on the 'Desktop screen' > Select 'nVidia Control Panel' > Configure 'Multiple Displays' as shown in the reference image below:
Not sure, what is meant by 'the second monitor not listed'. If you still face issues, a screenshot of the 'error message' will be useful.