That’s what the Vista license says. With XP, you can install it on a new computer as many times as you want (after removing it from the old one.)
In Vista, you will only be able to do this once. More than that, you have to buy a new copy of Vista. Even though your original version is quite legal, Microsoft has declared it to to be dead. What a bunch of greedy pigs. But since Microsoft has rather clearly run out of new ideas and innovation, they’ve decided the only way to keep going is to police everything and assume the user is the enemy. A shortsighted strategy, to be sure.
It gets worse. If the Redmond Borg thinks your copy of Vista is pirated, it’ll automatically lock down Vista so, for example, you can only web browse with IE and only for an hour. Gee, I bet that nasty piece of anti-piracy software won’t fail more than on, what, a few million legal users who the anti-piracy software mistakenly decided weren’t.
Yet more reasons to go to Linux.
(The MacBook is nice, but hey, the 13-inch version has so many random lockups that a class action lawsuit is coming.)