windows xp - Is it possible to drive 2 monitors from a video card that's designed for one?

Here is the situation. I've been given a PC at a client site. It has a single monitor, however, I'd like to have another one since my work would go much faster.

I've been told that, no, I can't add another video card. But I can plug into the box anything external. So is there an external device that I can buy that would allow me to add the extra monitor?