Hi Ellen..
To get more than two monitors hooked up to one computer, you either need:
1. As Baer pointed out, an A/B switch - this directs the signal to either monitor but not both
or
2. A dual-output video card - basically has two ports in the back to plug two monitors into
or
3. Two video cards - this one is tricky as you just can't go and buy any old video card, you have to get a card that fits an available slot, and it has to be new enough that Windows XP supports it as a second monitor
So for most people I recommend option 2, as the card usually comes with software that helps you set it up. Option 1 is the easiest, but you don't get a second desktop out of it, which is a nice place to put programs and stuff shown to customers when the computer isn't being used with IF.
Option three is certainly doable, but I highly recommend taking it to someone who knows how to do that. They have to open you box and check other slots as video cards come in several flavors (AGP, PCI, PCI express, etc.).
And finally, once it's installed you need to go to the Display Properties (R-click on the desktop and choose Properties) and enable the second monitor on the Settings tab. Easy if you know what you're doing, absolutely confusing if you don't.
I personally run three monitors at work.. gives me one massive 3072x768 desktop.
UltraMon, as Paul pointed out, is a very nice shareware tool to add features to the multiple monitor desktop that Microsoft left out.
And Paul is right, the other type is called a KVM (keyboard video mouse) switch that lets you run more than one computer on the same keyboard/mouse station. Very handy for testing, or if you have several servers that generally run unattended but occasionally need to get on them and install patches and stuff.