After downloading the Demo, and playing through the content in that, I bought a copy of the game to try it for 30 days. My question is, do I need to install the game from the disc, or will the key code in the box allow me to play the full game with the software already loaded?

Put the key in and launch the game. This will avoid your having to patch the game that is on the disk. It is my understanding that the demo is a full game program that has limitations built into it.

If this is not correct, you can load from disk as a second resort.

Actually, the Demo is the full client... To play the full game requires both a Retail package Key AND a subscription. (Either a Game Time Card Key, or a Credit Card Sub. If you decide you don't like the game, you can cancel the sub within the first 30 days of 'free' game play and your credit card will not be charged...)