I'm not 100% sure on this, but I believe that when you sign in using a Live account, anything you download is tied to it at least in the form of "purchases" (even if it's a free app). If you log into a Win8 machine with a Live account and download/install a bunch of apps from the Store, then go to another Win8 machine, sign in with Live account, you can go to the Store and it will show which apps you "purchased" using your account that are not installed on that machine, and it gives you the option to install it.
I recently went through this when I installed Windows 8 on my tablet. I've already been using my laptop with Windows 8 and downloaded a bunch of stuff from the Store. When I signed in with my Live account on my tablet and went to the Store, it showed me everything that was and wasn't installed on my tablet that I had downloaded before. Then I just selected all and installed!