To play this tonight? I reaaaaally don't feel like hunting for the disc, so I'd like to know what I need to do.

Can't you just download and install it from the Battle.net site?

@sarnecki: did you register the client online? you can download the game from battle.net so you won't need the disc. but to answer the thread title, yes, the game needs to be installed as it's essentially a patch.

I looked it up and no, you don't need the original disc. If you're buying a retail copy like I am, the original game is included on the Heart Of The Swarm disc, so for those wondering, no need to track it down.

Technically, they both run from the same client (It is just called Starcraft II). You can switch between expansions in the "Gameplay" settings in the options menu in the upper left corner of that area of the menu.

You don't want to install using your WoL disc, it will take like 8 hours to patch because it starts at 1.0 and then does a bunch of patches. It is much faster to use the HOTS disc or install the client from Battlenet.