Steam is neat in that you can copy your entire Steam folder IE c/programs/Steam whatever. Then on another comp put it where ever and launch steam. It might act funny at first but it will do some semi install thingy then it will work. CAUTION this does not include any game saves that are kept in C/MyDocuments or AppData folder.
If you do not want to copy whole steam folder you can copy the games you want to backup, reinstall steam on new comp/OS install and put the game folders in their respected locations.
For Valve games this is often Steam/SteamApps/ACCOUNT NAME for other games IE COD this will be Steam/SteamApps/Common
if you have more questions about Steam, or if I confused you pm me