my guess would just be a OEM copy of Vista Premium Upgrade. You will get a shinny sticker and a DVD Rom. If the upgrade process works like Xp's, you would insert your Vista disk, after some thinking it will ask you to prove previous ownership, insert your xp or 2k disk and it will (hopefully) like it. It will then go on to install normally. This is all a guess though...
From what I understand, the upgrade version will look for it's upgrade counterpart (XP home to Vista Basic, XP Pro to Vista Business, etc), load and then disable your current copy of XP.
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