Is there a way to transfer steam games from my laptop to my desktop without re-downloading?

I have a 1TB portable hard-drive and I want to know if I can "Copy Paste" the games onto it and move them to my desktop. The games are Portal 2 and Team Fortress 2 and I would rather not re-download as my download is incredibly slow. Is there a LEGAL way to do this? Help!?!?!