Hoping a Lurker has a solution.

[ZatarRufus]

Perhaps a Vista Downgrade?

<snip> The answer is that only one XP Professional license is required in order to activate any number of Vista downgrades. I'm sure there must be some ceiling you can reach at which point they'd stop you, but in my example, the Microsoft representative told me that one XP is good for the five Vista machines I'm going to acquire, and all they'd ask for when I call in is the Vista serial number. <snip>

And it appears Vista can still be purchased.