There are a few factors:
1) Your operating system
2) Your vid card software
Vista and XP have similar setup screens for dual monitor support. Access the display properties page and you can decide which monitor is your primary (has the task bar, start button, etc.) and which is the slave. You can do all sorts of adjustments in there. Some vid card software will let you do even more tweaking.
Like everyone said above, once everything is setup you just drap windows around like you normally would. It's very easy to hookup a 2nd monitor and use.
How to in Vista: [ame]http://www.youtube.com/watch?v=tu0kZeXWb-E[/ame]
How to in XP: [ame]http://www.youtube.com/watch?v=khzbzZn8AmE[/ame] (Ignore the resolution settings... set yours to your LCD native resolution)