You need a graphics (video) card that has 2 outputs. Connect one monitor to each of these outputs, and you have a dual monitor setup. Simple. The monitors can be of different sizes.
If you're running Windows XP and an nVidia card, you can use nVidia's "Control panel" program, the option "Set up multiple displays". You get there by clicking the green icon with the tooltip "nVidia settings" on the right side of the taskbar. It's easy, especially if the monitors have the same resolution.