If the Vista Business version used for the upgrade is a Retail version, then buying the legal copies and using their serial key in the current installs should work, and not require anything else.
If the version used was an Upgrade version, or an OEM version you'll need to find legal copies of that specific version for the keys to work. OEM versions are not as easy to get a hold of, but Upgrade versions and Retail versions should be able to be bought in any computer store.
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Phil AKA Vacunita
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Ignorance is not necessarily Bliss, case in point:
Unknown has caused an Unknown Error on Unknown and must be shutdown to prevent damage to Unknown.