Beware those frugal shoppers who are trying to upgrade to Vista on the cheap. As it turns out, Microsoft is not allowing those who choose to "Upgrade" their XP operating systems to the new Vista to keep their old version of XP. From the Vista EULA...
13. UPGRADES. To use upgrade software, you must first be licensed for the software that is eligible for the upgrade. Upon upgrade, this agreement takes the place of the agreement for the software you upgraded from. After you upgrade, you may no longer use the software you upgraded from.
I guess Microsoft views its own outdated, patched-to-hell operating system to be one of its competitors. If the upgrade screws up, you can't go back Jack!