right click on your desktop and select properties
go to the settings tab
if your display adapter can handle multiple displays, there should be a 1 box and a 2 box.
click on the 2 box and select "Extend my Windows desktop onto this monitor"
select the resolution, click apply or ok
as far as I know, this process is the same for xp and vista
you'll need 2 monitors (obvious enough) and the right cable. Chances are you'll have either 2 vga outputs or 1vga and 1 dvi. You'll need the appropriate cable as well as a monitor that can accept the cable (most pre-built brand name computers come with monitors that have vga inputs. Lower-end LCD monitors and all CRT monitors will only have vga)