Metro 2033 is a Steamworks game Im pretty sure. So Steam is required.
Sadly, If the game is used and has already been activate via Steam, you will need to buy a new copy. That one is no good.
Once the key is used, it can not be used again.
If the disc itself is bad, you can (if the key works) just register the key and Download through Steam, already patched.
Downloads are usually pretty fast deopending on your connection, and Metro isn't that big of a game.
I too am one who has been with Steam for quite sometime without any issues.
It may have something to do with the DVD drive unable to read the disc properly.