Microsoft has changed things with the upgrade versions of Windows Vista, When installing XP, you could insert a CD or DVD with Windows 98 or another upgradable Windows version on it. Not so with Vista! It requires you to have an upgradable Windows already installed on one of your disks. Now, I suppose you could install XP on a USB drive and carry it around with you or simply install XP first, before you install Vista but that’s really a pain.
WindowsSecrets.com has published this article that tells you how to install Vista on a bare hard drive, and the author (Brian Livingston) has come up with a reasonable rationalization as to why this approach is OK. The process is a little funky in that you boot the DVD and go partway through an install and then use that partially installed copy as the version you’re upgrading from, but it sure looks like it’ll work. And, when you think about it, if you blow you hard disk and have to reinstall from scratch, how much sense does it make to first have to install XP in order to upgrade to Vista again?