Exactly as it should have been.
You can't.
Irrelevant.
You need to purchase a separate Vista license for each computer on
which you install it. (As long as you have multiple identical licenses,
it doesn't matter if you use the same CD for the installations, as long
as you use a different license each time.)
Just as it has *always* been with *all* Microsoft operating systems,
it's necessary (to be in compliance with both the EULA and U.S.
copyright law
http://www4.law.cornell.edu/uscode/17/117.html), if not
technically) to purchase one WinXP license for each computer on which it
is installed. (Consult an attorney versed in copyright law to determine
final applicability in your locale.) The only way in which WinXP
licensing differs from that of earlier versions of Windows is that
Microsoft has finally added a copy protection and anti-theft mechanism,
Product Activation, to prevent (or at least make more difficult)
multiple installations using a single license.