Now, what about a Windows desktop. Here's a recent one from Gateway, and on the back you'll see monitor connections for DVI and HDMI. One will need to be used for your main display, while the other is connected to your second monitor. It doesn't matter which port gets connected to which display, but chances are that your older monitor will lack HDMI, while a newer monitor will be able to accommodate an HDMI connection.
If all you see back here is a single monitor connection, you could spend some money on a video card or even a USB adapter
. If it were me, though, I'd take it as a sign that I need to update my computer.