When you buy a game the game gets tied to THAT steam account it was bought on, so if you say format the games stays in that steam account. Let me explain more, when you format you loose ALL your data inclu; steam when you re-install steam and login to your steam account the games will still be there so you can double-click the game you want and install it agin, think of it as a friends computer, you install steam, login to your account and install the game you want.
hope this helped. ask me if you need more help
Its not much of a video tutorial but i thought you might learn better in writing than following video