Do you mean:

Questions:
1) Is it compatible?
or
2) Can you use the same key?

Answers:
1) Yes!
and
2) As long as it is only being used on one PC, you ARE allowed (though some copies are multiple user licenses - check your box) - if the key does not work, you can phone Microsoft and they will help you get a new key.


As a final bit of information - Why bother getting Vista? W7 will be out soon and it's SO much better than Vista!! Go try the Beta version from the Microsoft site here!