Also, some graphics cards come built-in with the capability to have more than one monitor interface.
You can always get another graphics card, and the mutli-monitor capability of XP will kick in. You would then do what Hero suggested (go to "Settings" Tab). And you'll find a drop-down list of all current monitors connected to your computer. Select the one you just attached, and click "Extend my windows desktop onto this monitor".
It will then extend the desktop onto that monitor.