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You have to open "display properties" and choose to extend your desktop onto that monitor.
Make sure you unplug it and plug it back in, and see if it recognizes it as a monitor (It'll make a sound like you plugged in a usb device). If it makes three quick noises that are all low-pitched, it's likely your hardware requires some kind of drivers that aren't available. It's possible that it's your video card that needs drivers in order for the S-video-out to work.
Holdthe Windows key (between CTRL and ALT) and "Pause|Break", and it should open your system properties.
In there, go to the Device manager (I believe in Win7, it's on the left side near the top). Check for anything with an X or exclamation over it. Also, check in "Display Adapters" and see if your other monitor is listed there.
Hope this helps!
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