SR-71 Blackbird said:
You would need a different key for each installation of 7.
Although that is usually the case there are versions of xp, 7, and I believe 8 as well that include multiple installs per key ex. family editions something like 3 or 5 uses out of 1 key, business edition not sure how many but a lot more, and some corporate editions have unlimited uses. But they cost a huge load o' change, and are hard to get.