While I don't know for sure, this is how it worked with Windows XP. You can activate a bunch of times online, on different machines. Once you do it enough, you have to call them and talk to one of the reps. I've done this numerous times with the same copy as I upgraded my PC. They ask simple questions like "Is this the first time installing the product?" I always answer with the most simple answer possible such as "Nope". A few seconds later, they read off the code you need to put in and then you can reactivate online a bunch of more times. Wash, rinse, repeat.
Again, not sure if this applies to Vista.