Possible Duplicate:
How do I move Steam games to a new computer without re-downloading them?
Recently, I bought Portal 2 from Steam. I downloaded and installed it on my PC, which is running Windows. Unfortunately, days later, my PC crashed, and it was rendered useless. Thankfully, my technician helped me transfer everything in my hard drives to a back up drive. Yesterday, I got a new PC, which runs Windows too. Could I transfer the game files from the back up drive to my new PC without buying the game again? It's okay if my game progress is lost in the process, because I don't want to spend more money buying the game again.