It might be easier to use the backup function in STEAM to archive the files to a DVD or Dual Layer DVD.
There are so many files updated I don't think you could make a new install.
It's probably better to filecopy (or archive) the folder where all the STEAM files are to a DVD. This way you get all the updates too. When you want to reinstall, just filecopy/unarchive it back to you hard drive. I think you have to delete the ClientRegistry.blob file before running STEAM as this stores all the paths of the previous install. When you log into your STEAM account it will create a new ClientRegistry.blob file and you will be able to play the game again. It might also apply any updates you missed in the meantime so you may have to back it up again
